Glossary

ACK A 1-bit flag in the TCP header that indicates the acknowledgement field is valid.

acknowledgement A notification sent from one network device to another to confirm that some event (for example, receipt of a message) has occurred.

acknowledgement number A 32-bit field in the TCP segment header that specifies the sequence number of the next byte this host expects to receive as a part of the TCP session. It is used to recognize lost packets and flow control.

Address Resolution Protocol (ARP) The method for finding a host’s hardware address from its IPv4 network layer address.

administrative distance A rating of the trustworthiness of a routing information source. In Cisco routers, administrative distance is expressed as a numerical value between 0 and 255. The higher the value, the lower the trustworthiness rating.

American National Standards Institute (ANSI) A voluntary organization comprised of corporate, government, and other members that coordinates standards-related activities, approves U.S. national standards, and develops positions for the United States in international standards.

American Standard Code for Information Interchange (ASCII) A 8-bit code for character representation (7 bits plus parity).

amplitude modulation (AM) Modulation technique whereby information is conveyed through the amplitude of the carrier signal. Compare with FM and PAM.

AND One of three basic binary logic operations. ANDing yields the following results: 1 AND 1 = 1, 1 AND 0 = 0, 0 AND 1 = 0, 1 AND 0 = 0.

appliance-based firewalls Hardware device designed to slow down or stop cyber-terrorists, hackers, DoS attackers, and malicious viruses from infiltrating a network or individual computer.

Application layer Layer 7 of the OSI reference model. This layer provides services to application processes (such as electronic mail, file transfer, and terminal emulation) that are outside of the OSI model. The application layer identifies and establishes the availability of required communications resources.

argument Additional data that is provided with a command to provide information used by the execution of the command. IOS command arguments are entered at the command-line interface (CLI) after the command.

ARP cache A logical storage in a host’s RAM to store ARP entries. See also ARP table.

ARP poisoning A technique used to attack an Ethernet network by sending fake ARP messages to an Ethernet LAN. These frames contain false MAC addresses that “confuse” network devices, such as switches. As a result, frames intended for one node can be mistakenly sent to another node. See also ARP spoofing.

ARP spoofing A technique used to attack an Ethernet network by sending fake ARP messages to an Ethernet LAN. These frames contain false MAC addresses that “confuse” network devices, such as switches. As a result, frames intended for one node can be mistakenly sent to another node. See also ARP poisoning.

ARP table A logical storage in a host’s RAM to store ARP entries. See also ARP cache.

asynchronous Communication that does not use a common clock between the sender and receiver. To maintain timing, additional information is sent to synchronize the receive circuit to the incoming data. For Ethernet at 10 Mbps, the Ethernet devices do not send electrical signals for synchronization.

automatic medium-dependent interface crossover (AMDIX) A version of the medium dependent interface (MDI) enabling a connection between corresponding devices. An MDI port or uplink port is a port on a switch, router, or network hub connecting to another switch or hub using a straight-through cable rather than an Ethernet crossover cable.

auxiliary (AUX) port The purpose of the auxiliary port is for connecting to an external modem. Once configured, this modem can be used as a backup demand-dial connection to another location, or as a way to dial in to the router for troubleshooting purposes should regular connectivity fail.

banner motd To configure the message-of-the-day (MOTD) banner that displays when the user logs in to a Cisco device, use the banner motd command. To revert to the default, use the no form of this command.

banners Banners are informational messages that can be displayed to users. See banner motd for an example.

binary notation A number system having a base of two, numbers being expressed by sequences of the digits 0 and 1: used in computing, as 0 and 1 can be represented electrically as off and on.

bits Binary digits used in the binary numbering system. A bit can be 0 or 1.

branch routers Branch routers come in two categories: general purpose and integrated services. General purpose routers typically focus on basic wide area network (WAN) routing, supporting a limited number of routing protocols and a variety of WAN interfaces. General purpose routers can be a good choice when one is interested in simple routing. A typical use case arises when customers need Layer 3 visibility with a full Internet routing table for a branch network with multiple WAN exit points, or when the service provider requires the customer to peer with their network. An integrated services router (ISR) allows the customer to take advantage of advanced technologies, enabling them to more effectively meet their strategic business challenges. It provides the option of telephony (VoIP) and streaming video, as well as WAN routing.

Bring Your Own Device (BOYD) The policy of permitting employees or students to bring personally owned mobile devices (laptops, tablets, and smartphones) to their workplace or school, and use those devices to access privileged company/school information and applications.

broadcast Data packet that will be sent to all nodes on a network. Broadcasts are identified by a broadcast address. Compare with multicast and unicast.

broadcast address A logical address at which all devices connected to a multiple-access communications network are enabled to receive datagrams. A message sent to a broadcast address is typically received by all network-attached hosts, rather than by a specific host.

broadcast MAC address Special address reserved for sending a message to all stations. Generally, a broadcast address is a MAC destination address of all ones.

broadcast transmission See broadcast, unicast, and multicast.

bus Common physical signal path composed of wires or other media across which signals can be sent from one part of a computer to another. Bus is also known as highway.

carrier sense multiple access with collision avoidance (CSMA/CA) A mechanism used to regulate the transmission of data onto a network medium. CSMA/CA is similar to CSMA/CD except the devices first request the right to send, which hopefully avoids collisions. CSMA/CA is used in 802.11 WLANs.

carrier sense multiple access with collision detection (CSMA/CD) The MAC algorithm used by Ethernet devices in a shared media. The protocol requires a node wishing to transmit to listen for a carrier signal before trying to send. If a carrier is sensed, the node waits for the transmission in progress to finish before initiating its own transmission. If a collision occurs and is detected, the sending node uses the backoff algorithm before retransmitting.

cellular A radio network distributed over land areas called cells, each served by at least one fixed-location transceiver, known as a cell site or base station. In a cellular network, each cell uses a different set of frequencies from neighboring cells to avoid interference and provide guaranteed bandwidth within each cell.

central equipment room Room where all networking equipment is located, normally in 19″ equipment racks.

Cisco IOS (Internetwork Operating System) Software used on most Cisco Systems routers and current Cisco network switches. (Earlier switches ran CatOS.) IOS is a package of routing, switching, internetworking and telecommunications functions integrated into a multitasking operating system.

class A Network address class that contains all addresses in which the most significant bit is zero. The network number for this class is given by the next 7 bits, therefore accommodating 128 networks in total, including the zero network, and including the existing IP networks already allocated.

class B Network address class in which all addresses have the two most-significant bits set to 1 and 0. For these networks, the network address is given by the next 14 bits of the address, thus leaving 16 bits for numbering host on the network for a total of 65,536 addresses per network.

class C Network address class in which the 3 high-order bits are set to 1, 1, and 0, and designating the next 21 bits to number the networks, leaving each network with 256 local addresses.

classless addressing An IPv4 addressing scheme that uses a subnet mask that does not follow classful addressing limitations. It provides increased flexibility when dividing ranges of IP addresses into separate networks. Classless addressing is considered the best in current network implementations.

CLI prompt Cisco IOS have two basic command-line interface prompts: the user mode prompt (>) and the privileged mode prompt (#).

cloud computing Computing provided as a service.

coaxial cable Cable consisting of a hollow outer cylindrical conductor that surrounds a single inner wire conductor. The cable has three different layers of material surrounding the inner conducting material: the outer conductor, the insulator, and the protective outer jacket.

collaboration tools Software that helps people collaborate. Anything that helps to solve a predefined task together as a group is an effective collaboration tool.

command The statements you use to configure Cisco devices.

command reference Document that contains Cisco IOS commands that are supported in many different software releases and on many different platforms.

connectionless Term used to describe data transfer without the existence of a virtual circuit. Compare with connection-oriented.

connection-oriented Term used to describe data transfer that requires the establishment of a virtual circuit.

console password CLI command that allows you to password-protect the console port.

console port DTE through which commands are entered into a host. It is an out-of-band management port. See also out-of-band access.

contention-based access Access method in which network devices compete for permission to access the physical medium. Contrast with circuit switching and token passing.

context-sensitive help The use of the question mark (?) while configuring Cisco Devices to receive help about the command syntax to be used.

controlled access Selective restriction of access to a place or a resource.

converged network The efficient coexistence of telephone, video, and data communications within a single network.

copper cable The use of copper cable network connectivity versus wireless or fiber connectivity.

crosstalk Interfering energy transferred from one circuit to another.

CSMA/Collision Avoidance See carrier sense multiple access with collision avoidance (CSMA/CA).

CSMA/Collision Detection See carrier sense multiple access with collision detection (CSMA/CD).

cut-through switching Switches operating in cut-through switching mode start forwarding the frame as soon as the switch has read the destination details in the packet header. A switch in cut-through mode forwards the data before it has completed receiving the entire frame.

data encapsulation The process by which a device adds networking headers and trailers to data from an application for the eventual transmission of the data onto a transmission medium.

data field A specific place to store data, such as a column in a database or field in a data entry form.

data link address The physical address that is burned into the network interface card (MAC address).

Data link layer Layer 2 of the OSI reference model. This layer provides reliable transit of data across a physical link. The data link layer is concerned with physical addressing, network topology, line discipline, error notification, ordered delivery of frames, and flow.

datagrams A basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order of arrival are not guaranteed by the network service.

dedicated leased line Communications line that is indefinitely reserved for transmissions. A dedicated leased line is always activate, rather than switched as transmission is required.

de-encapsulation A process by which an end device, after it receives data over some transmission medium, examines the headers and trailers at each successive higher layer, eventually handing the data to the correct application.

default gateway A device on a network that serves as an access point to other networks. A default gateway is used by a host to forward IP packets that have destination addresses outside the local subnet. A router interface typically is used as the default gateway. When the computer needs to send a packet to another subnet, it sends the packet to its default gateway. Also known as default router.

denial of service (DoS) A type of attack whose goal is to cause problems by preventing legitimate users from being able to access services, thereby preventing the normal operation of computers and networks.

destination IP address The Layer 3 address to which the data is going.

destination IP address field The field that houses the Layer 3 IP address to which the data is going.

destination MAC address field The field that houses the Layer 2 MAC address to which the data is going.

DHCP (Dynamic Host Configuration Protocol) A protocol used to dynamically assign IP configurations to hosts. The services defined by the protocol are used to request and assign an IP address, default gateway, and DNS server address to a network host.

dial-up A form of Internet access that uses the facilities of the public switched telephone network (PSTN) to establish a dialed connection to an Internet service provider(ISP) via telephone lines. The user’s computer or router uses an attached modem to encode and decode Internet Protocol packets and control information into and from analogue audio frequency signals, respectively.

differentiated services (DS field) Differentiated services or DiffServ is a computer networking architecture that specifies a simple, scalable and coarse-grained mechanism for classifying and managing network traffic and providing quality of service (QoS) on modern IP networks. DiffServ can, for example, be used to provide low-latency to critical network traffic such as voice or streaming media while providing simple best-effort service to non-critical services such as web traffic or file transfers.

directly connected routes A router can route packets to networks that are directly connected to it without running a routing protocol. Directly connected routes are loaded into the routing table by default.

disruption of service Denial or disruption of service attacks affect the availability of data, services, and network elements. For the most part, service disruptions caused by computer intruders have been brought about by accidental actions. Unintentional disruptions caused by computer intruders are much more common than malicious disruptions.

DNS (Domain Name System) An application layer protocol used throughout the Internet for translating host names into their associated IP addresses.

DNS server An Internet-wide system by which a hierarchical set of DNS servers collectively hold all the name-IP address mappings, with DNS servers referring users to the correct DNS server to successfully resolve a DNS name.

DSL (digital subscriber line) Public network technology that delivers high bandwidth over conventional telco local-loop copper wiring at limited distances. Typically used as an Internet access technology, connecting a user to an ISP.

Dual in-line memory module (DIMM) A DIMM is a double SIMM (single in-line memory module). Like a SIMM, it’s a module containing one or several random access memory (RAM) chips on a small circuit board with pins that connect it to the computer motherboard. A SIMM typically has a 32 data bit (36 bits counting parity bits) path to the computer that requires a 72-pin connector.

dual stack Dual stack means that devices are able to run IPv4 and IPv6 in parallel. It allows hosts to simultaneously reach IPv4 and IPv6 content, so it offers a very flexible coexistence strategy.

duplex settings Two types of settings used for communications on networks: half duplex and full duplex. A half duplex setting allows for communication in both directions, but only one direction at a time. A walkie-talkie is an example of a half duplex system. Full duplex would allow communication in both directions. The landline telephone is a good example of full duplex.

Dynamic Host Configuration Protocol for IPv6 (DHCPv6) A protocol used to dynamically assign IPv6 configurations to hosts. The services defined by the protocol are used to request and assign an IP address, default gateway, and DNS server address to a network host.

dynamic random-access memory (DRAM) A type of semiconductor memory in which the information is stored in capacitors on a Metal Oxide Semiconductor (MOS) integrated circuit. Typically each bit is stored as an amount of electrical charge in a storage cell consisting of a capacitor and a transistor. Due to leakage the capacitor discharges gradually and the memory cell loses the information. Therefore, to preserve the information, the memory has to be refreshed periodically.

dynamic window sizes A new technique for TCP implementations to dynamically and automatically determine the best window size for optimum network performance. This technique results in greatly improved performance, a decrease in packet loss under bottleneck conditions, and greater control of buffer utilization by the end hosts.

electromagnetic interference (EMI) Interference by magnetic signals caused by the flow of electricity. EMI can cause reduced data integrity and increased error rates on transmission channels. The physics of this process are that electrical current creates magnetic fields, which in turn cause other electrical currents in nearby wires. The induced electrical currents can interfere with proper operation of the other wire.

enable password Unencrypted password used to allow access to privileged EXEC mode from IOS user EXEC mode.

Enable secret Encrypted password used to limit access to privileged EXEC mode from IOS user EXEC mode.

end device A device such as a desktop or mobile device that is used by an end user.

enterprise networks An enterprise network is an enterprise’s communications backbone that helps connect computers and related devices across departments and workgroup networks, facilitating insight and data accessibility. An enterprise network reduces communication protocols, facilitating system and device interoperability, as well as improved internal and external enterprise data management.

entrance facilities Entrance facility refers to the entrance to a building for both public and private network service cables (including antenna transmission lines, where applicable), including the entrance point at the building wall or floor, and continuing to the entrance room or entrance space.

Ethernet Baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation. Ethernet networks use CSMA/CD and run over a variety of cable types at 10 Mbps. Ethernet is similar to the IEEE 802.3 series.

Ethernet crossover An Ethernet crossover cable is a type of Ethernet cable used to connect computing devices together directly. Normal straight through or patch cables were used to connect from a host network interface controller (a computer or similar device) to a network switch or hub. A cable with connections that “cross over” was used to connect two devices of the same type: two hosts or two switches to each other. Owing to the inclusion of Auto-MDIX capability, modern implementations of the Ethernet over twisted pair standards usually no longer require the use of crossover cables.

Ethernet LAN interfaces Ethernet Networking Interface, or ENI, allows any computer on Ethernet network to access controllers allowing users to send production data, alarm messages, or status information to computers, cellular phones, or pagers capable of receiving email. The ENI module allows companies to leverage existing cable, hubs, switches, and routers already installed in facilities.

Ethernet LLC sublayer In the seven-layer OSI model of computer networking, the logical link control (LLC) data communication protocol layer is the upper sublayer of the data link layer, which is itself layer 2. The LLC sublayer provides multiplexing mechanisms that make it possible for several network protocols (IP, IPX, Decnet and Appletalk) to coexist within a multipoint network and to be transported over the same network medium. It can also provide flow control and automatic repeat request (ARQ) error management mechanisms. The LLC sublayer acts as an interface between the media access control (MAC) sublayer and the network layer.

Ethernet Straight-through A type of twisted pair copper wire cable for local area network (LAN) use for which the RJ-45 connectors at each end have the same pinout (i.e., arrangement of conductors). It is identical to crossover cable, except that in the latter the wires on the cable are crossed over so that the receive signal pins on the connector on one end are connected to the transmit signal pins on the connector on the other end. Straight-through cable is also commonly referred to as patch cable. However, this might be confusing in some situations because patch cable also has a broader definition that emphasizes the fact that there is a connector on each end rather than the equality (or lack thereof) of the pinouts. Straight-through cable is used to connect computers and other end-user devices (e.g., printers) to networking devices such as hubs and switches. It can also be used to directly connect like devices (e.g., two hubs or two switches) if the cable is plugged into an uplink port on one (but not both) of the devices. Crossover cable is used to connect two like devices without the use of an uplink port.

Exec Timeout To configure the inactive session timeout on the console port or the virtual terminal, use the exec-timeout command. To revert to the default, use the no form of this command.

expandability To increase the size, volume, quantity, or scope of.

expectational acknowledgement Acknowledgement used by TCP where the ACK number sent back to the source to indicate the next byte that the receiver expects to receive.

experimental addresses One major block of addresses reserved for special purposes is the IPv4 experimental address range 240.0.0.0 to 255.255.255.254. Currently, they cannot be used in IPv4 networks. However, these addresses could be used for research or experimentation.

extended star A network topology characterized by a central location connected to multiple hubs. In an extended star, these interconnected hubs may be connected to more hubs. It is essentially a hierarchical topology but typically is drawn with the central site in the center, with the rest of the topology radiating outward in all directions. This is sometimes called a hierarchical star.

extranet Part of a company’s intranet that is extended to users outside the company (that is, normally over the Internet).

fault tolerance The design on networks that can continue to operate without interruption in the case of hardware, software, or communications failures.

fiber-optic cable Physical medium that uses glass or plastic threads to transmit data. A fiber-optic cable consists of a bundle of these threads, each of which is capable of transmitting data into light waves.

FIN A 1-bit field in the TCP header that is used by a device that wants to terminate its session with the other device. This is done by inserting the FIN flag in the flag field found in the TCP segment.

fixed configuration switches A switch that is the opposite of a modular switch; you cannot add another module to a fixed configuration switch.

flash A removable component that has memory space for storage. Used on the router or switch for storing the compressed operating system image.

flat network design A flat network is a network in which all workstations are directly connected to each other, except for the presence of switches and can communicate without the need for intermediate devices such as routers. A flat network is one without subnets, as a result, its topology is not divided into layers or modules. Every station on a flat network receives a copy of every message sent. Security is poor and it is not possible to establish alternative paths to destinations.

flow control The management of data flow between devices in a network. It is used to avoid too much data arriving before a device can handle it, causing data overflow.

Flow label field Part of the IPv6 header, originally created for giving real-time applications special service. The flow label when set to a non-zero value now serves as a hint to routers and switches with multiple outbound paths that these packets should stay on the same path so that they will not be reordered. It has further been suggested that the flow label field be used to help detect spoofed packets

fragmentation The dividing of IP datagrams to meet the MTU requirements of a Layer 2 and Layer 3 protocol.

fragment-free switching One of three internal processing options on some Cisco LAN switches in which the first bits of the frame can be forwarded before the entire frame is received, but not until the first 64 bytes of the frame are received, in which case, in a well-designed LAN, collision fragments should not occur as a result of this forwarding logic.

frame The Layer 2 PDU that has been encoded by a data link layer protocol for digital transmission. Some different kinds of frames are Ethernet frames and PPP frames.

Frame Check Sequence A field in many data link trailers used as part of the error detection process.

frame encoding technique Refers to the many encoding techniques available to compress frames. MPEG coding is an example.

frequency modulation (FM) the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. (Compare with amplitude modulation, in which the amplitude of the carrier wave varies, while the frequency remains constant.)

FTP (File Transfer Protocol) A standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet.

FTTH (fiber-to-the-home) and Access Networks Fiber reaches the boundary of the living space, such as a box on the outside wall of a home. Passive optical networks and point-to-point Ethernet are architectures that deliver triple-play services over FTTH networks directly from an operator’s central office.

full duplex Communication that allows receipt and transmission simultaneously. A station can transmit and receive at the same time. There are no collisions with full-duplex Ethernet transmission.

Gigabit Ethernet The common name for all the IEEE standards that send data at 1 gigabit per second.

Global configuration mode To configure a Cisco network device you must enter the Global Configuration operating mode. After passing thru the User EXEC and Privilege EXEC modes you enter the Global Configuration mode by entering the configure command.

global routing prefix An IPv6 prefix that defines an IPv6 address block made up of global unicast addresses, assigned to one organization, so that the organization has a block of globally unique IPv6 addresses to use in its network.

header In computer networking, a set of bytes placed in front of some other data, encapsulating that data, as defined by a particular protocol.

Hexadecimal Numbering (Base 16) A number representation using the digits 0 through 9, with their usual meaning, plus the letters A through F to represent hexadecimal digits with values of 10 to 15. The right-most digit counts ones, the next counts multiples of 16, then 16^2=256.

Hop limit field See Time-to-Live (TTL) field.

horizontal cabling IW (inside wiring) or Plenum Cabling and connects telecommunications rooms to individual outlets or work areas on the floor, usually through the wireways, conduits or ceiling spaces of each floor.

host address The IP address assigned to a computer.

host device Any device on a network that has an IP address assigned to it.

host name The alphanumeric name of an IP host.

HTTP (Hypertext Transfer Protocol) The protocol used by web browsers and web servers to transfer files, such as text and graphic files.

HTTP Secure (HTTPS) protocol A communications protocol for secure communication over a computer network, with especially wide deployment on the Internet. Technically, it is not a protocol in and of itself; rather, it is the result of simply layering the Hypertext Transfer Protocol (HTTP) on top of the SSL/TLS protocol, thus adding the security capabilities of SSL/TLS to standard HTTP communications.

hub and spoke The hub-and-spoke distribution paradigm (or model or network) is a system of connections arranged like a chariot wheel, in which all traffic moves along spokes connected to the hub at the center. The model is commonly used in industry, in particular in transport, telecommunications, and freight, as well as in distributed computing.

IEEE 802.11 A set of medium access control (MAC) and physical layer (PHY) specifications for implementing wireless local area network (WLAN) computer communication in the 2.4, 3.6, 5 and 60 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee (IEEE 802). The base version of the standard was released in 1997 and has had subsequent amendments. The standard and amendments provide the basis for wireless network products using the Wi-Fi brand. Although each amendment is officially revoked when it is incorporated in the latest version of the standard, the corporate world tends to market to the revisions because they concisely denote capabilities of their products. As a result, in the market place, each revision tends to become its own standard.

IEEE 802.15 A working group of the Institute of Electrical and Electronics Engineers (IEEE). IEEE 802 standards committee which specifies Wireless Personal Area Network (WPAN) standards. It includes seven task groups.

IEEE 802.16 A series of Wireless Broadband standards written by the Institute of Electrical and Electronics Engineers (IEEE). The IEEE Standards Board established a working group in 1999 to develop standards for broadband for Wireless Metropolitan Area Networks. The Workgroup is a unit of the IEEE 802 local area network and metropolitan area network standards committee.

IMAP, SMTP, POP (email) email protocols IMAP (Internet Message Access Protocol) is a standard protocol for accessing email from your local server. IMAP is a client/server protocol in which e-mail is received and held for you by your Internet server. The SMTP (Simple Mail Transfer Protocol) protocol is used by the Mail Transfer Agent (MTA) to deliver your email to the recipient’s mail server. The SMTP protocol can only be used to send emails, not to receive them. The POP (Post Office Protocol) protocol provides a simple, standardized way for users to access mailboxes and download messages to their computers.

inband router interfaces The router interface you Telnet through. A way to connect to a Cisco router is in-band, through the program Telnet. Telnet is a terminal emulation program that acts as though it’s a dumb terminal. You can use Telnet to connect to any active interface on a router, such as an Ethernet or serial port.

information theft The process of obtaining data from a network you are not authorized to have access to.

infrastructure In information technology and on the Internet, infrastructure is the physical hardware used to interconnect computers and users. Infrastructure includes the transmission media, including telephone lines, cable television lines, and satellites and antennas, and also the routers, aggregators, repeaters, and other devices that control transmission paths. Infrastructure also includes the software used to send, receive, and manage the signals that are transmitted.

initial sequence number (ISN) Refers to the unique 32-bit sequence number assigned to each new connection on a Transmission Control Protocol (TCP)-based data communication. It helps with the allocation of a sequence number that does not conflict with other data bytes transmitted over a TCP connection. An ISN is unique to each connection and separated by each device.

inoculation The action of inoculating or vaccinating your network against network threats.

instant messaging (IM) Real-time communication between two or more people through text. The text is conveyed through computers connected over a network such as the Internet. Files can also be transferred through the IM program to share files. An example of an IM program is Microsoft Messenger.

Institute of Electrical and Electronics Engineers (IEEE) An international, nonprofit organization for the advancement of technology related to electricity. IEEE maintains the standards defining many LAN protocols.

integrated firewalls Firewalls that integrate the world’s most proven firewall with a robust suite of highly integrated, security services for networks of all sizes: small and midsize business with one or a few locations, large enterprises, service providers, and mission-critical data centers.

intermediary devices For communication to run smoothly across the network there are devices that place intermediary roles in networking. These intermediary devices provide connectivity and work behind the scenes to ensure that data flows across the network. These devices connect the individual hosts (end devices) to the network and can connect multiple individual networks to form an internetwork. Examples of intermediary devices are routers, switches, hubs, wireless access point, servers, modems, and security devices.

International Organization for Standardization (ISO) An international standards body that defines many networking standards. Also, the standards body that created the OSI model.

International Telecommunication Union (ITU) Organization responsible for issues that concern information and communication technologies. The ITU coordinates the shared global use of the radio spectrum, promotes international cooperation in assigning satellite orbits, works to improve telecommunication infrastructure in the developing world, and assists in the development and coordination of worldwide technical standards.

Internet The network that combines enterprise networks, individual users, and ISPs into a single global IP network.

Internet Architecture Board (IAB) The committee charged with oversight of the technical and engineering development of the Internet by the Internet Society (ISOC).

Internet Assigned Numbers Authority (IANA) An organization that assigns the numbers important to the proper operation of the TCP/IP protocol and the Internet.

Internet Corporation for Assigned Names and Numbers (ICANN) A nonprofit private organization headquartered in the Playa Vista section of Los Angeles, CA that was created on September 18, 1998, and incorporated on September 30, 1998 to oversee a number of Internet-related tasks previously performed directly on behalf of the U.S. government by other organizations, notably the IANA, which ICANN now operates. Major responsibility is coordination of the IP address spaces (IPv4 and IPv6) and assignment of address blocks to regional Internet registries, for maintaining registries of Internet protocol identifiers, and for the management of the top-level domain name space (DNS root zone), which includes the operation of root name servers.

Internet Engineering Task Force (IETF) The standards body responsible for the development and approval of TCP/IP standards.

IP (Internet Protocol) Network layer protocol in the TCP/IP stack offering a connectionless internetwork service. IP provides features for addressing, type-of-service specification, fragmentation and reassembly, and security. Documented in RFC 791.

Internet Protocol version 4 (IPv4) In IP Version 4 (IPv4), a 32-bit address assigned to hosts using TCP/IP. Each address consists of a network number, an optional subnetwork number, and a host number. The network and subnetwork numbers together are used for routing, and the host number is used to address an individual host within the network or subnetwork.

Internet Protocol version 6 (IPv6) In IP Version 6 (IPv6), a 128-bit address assigned to hosts using TCP/IP. Addresses use different formats, commonly using a routing prefix, subnet, and interface ID, corresponding to the IPv4 network, subnet, and host parts of an address.

Internet Research Task Force (IRTF) Focuses on longer term research issues related to the Internet whereas the parallel organization, the IETF, focuses on the shorter term issues of engineering and standards making. The IRTF promotes research of importance to the evolution of the Internet by creating focused, long-term research groups working on topics related to Internet protocols, applications, architecture, and technology.

Internet Society (ISOC) An international, nonprofit organization founded in 1992 to provide leadership in Internet-related standards, education, and policy. It states that its mission is “to assure the open development, evolution and use of the Internet for the benefit of all people throughout the world.” The Internet Society has offices near Washington, DC and in Geneva, Switzerland. It has a membership base comprising more than 130 organizational and more than 55,000 individual members. Members also form “chapters” based on either common geographical location or special interests. There are currently more than 90 chapters around the world.

Internetwork Operating System (IOS) Cisco Internetwork Operating System software that provides the majority of a router’s or switch’s features, with the hardware providing the remaining features.

intranet A corporate system such as a website that is explicitly used by internal employees. Can be accessed internally or remotely.

IPv4 address A 32-bit number, written in dotted decimal notation, used by the IP to uniquely identify an interface connected to an IP network. It is also used as a destination address in an IP header to allow routing and as a source address to allow a computer to receive a packet and to know which IP address to send a response.

IP header The header defined by the IP. Used to create IP packets by encapsulating data supplied by a higher-layer protocol (such as TCP) behind an IP header.

IP Telephony IP telephony (Internet Protocol telephony) is a general term for the technologies that use the Internet Protocol’s packet-switched connections to exchange voice, fax, and other forms of information that have traditionally been carried over the dedicated circuit-switched connections of the public switched telephone network (PSTN).

ipconfig A console application in Microsoft Windows that displays all current TCP/IP network configuration values and can modify Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS) settings. In most cases, the ipconfig command is used with the command-line switch /all. This results in more detailed information than ipconfig alone.

IPv6 global unicast addresses The global unicast address is globally unique in the Internet.

IPv6 link-local address A link-local address is an IP address that is intended only for communications within the segment of a local network (a link) or a point-to-point connection that a host is connected to. Routers do not forward packets with link-local addresses.

Layer 2 LAN switch Layer 2 switching uses the media access control address (MAC address) from the host’s network interface cards (NICs) to decide where to forward frames. Layer 2 switching is hardware based, which means switches use application-specific integrated circuit (ASICs) to build and maintain filter tables (also known as MAC address tables or CAM tables). One way to think of a Layer 2 switch is as a multiport bridge.

Layer 3 switching Layer 3 switching is all hardware-based packet forwarding, and all packet forwarding is handled by hardware ASICs. Layer 3 switches really are no different functionally than a traditional router and perform the same functions, making routing decisions based on Layer 3 addresses (IP addresses, or logical addresses).

length field This 16-bit field, part of the IP header, defines the entire packet (fragment) size, including header and data, in bytes. The minimum-length packet is 20 bytes (20-byte header + 0 bytes data) and the maximum is 65,535 bytes—the maximum value of a 16-bit word. The largest datagram that any host is required to be able to reassemble is 576 bytes, but most modern hosts handle much larger packets. Sometimes subnetworks impose further restrictions on the packet size, in which case datagrams must be fragmented. Fragmentation is handled in either the host or router in IPv4.

link-local address An IPv4 address in the range of 169.254.1.0 to 169.254.254.255. Communication using these addresses is used with a TTL of 1 and limited to the local network.

local-area network (LAN) A network created for devices located in a limited geographic area, through which the company owning the LAN has the right to run cables.

local default route A local default route of a computer that is participating in computer networking is the packet-forwarding rule (route) taking effect when no other route can be determined for a given IP destination address. All packets for destinations not established in the routing table are sent via the default route. This route generally points to another router, which treats the packet the same way: If a route matches, the packet is forwarded accordingly; otherwise the packet is forwarded to the default route of that router. The process repeats until a packet is delivered to the destination, or not finding the route (or default route) and packet is dropped by the router.

local host Local host means this computer. It is a hostname that the computer’s software and users may employ to access the computer’s own network services via its loopback network interface. On most computer systems, local host resolves to the address 127.0.0.1, which is the most-commonly used IPv4 loopback address, and to the IPv6 loopback address ::1. Using the loopback interface bypasses local network interface hardware. The local loopback mechanism is useful for programmers to test software during development independent of any networking configurations. If a computer has been configured to provide a website, directing its web browser to http://localhost may display its home page.

Logical Link Control (LLC) The IEEE 802.2 standard that defines the upper sublayer of the Ethernet Layer 2 specifications (and other LAN standards).

logical ring topology The Token Ring is the most common example of a network with different logical and physical topologies. Here, the physical topology is a star bus; that is, there is a length of cable from each computer that connects it to a central hub (called a Multi-Station Access Unit, or MSAU). Inside the hub, however, the ports are wired together sequentially in a ring, and they send data around the ring instead of sending it out to all ports simultaneously as it would if the network were a logical star.

logical topology A map of the devices on a network representing how the devices communicate with each other.

loopback A special reserved IPv4 address, 127.0.0.1, that can be used to test TCP/IP applications. Packets sent to 127.0.0.1 by a computer never leave the computer or even require a working NIC. Instead, the packet is processed by IP at the lowest layer and is then sent back up the TCP/IP stack to another application on that same computer.

MAC address table On a switch, a table that lists all known MAC addresses, and the bridge/switch port out which the bridge/switch should forward frames sent to each MAC address.

MAC sublayer The MAC sublayer provides addressing and channel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multiple access network that incorporates a shared medium, e.g. Ethernet. The hardware that implements the MAC is referred to as a medium access controller.

Mail Delivery Agent (MDA) A computer software component that is responsible for the delivery of email messages to a local recipient’s mailbox. Also called an LDA, or local delivery agent. Within the Internet mail architecture, local message delivery is achieved through a process of handling messages from the message transfer agent, and storing mail into the recipient’s environment (typically a mailbox).

Mail Transfer Agent (MTA) Within Internet message handling services (MHS), software that transfers electronic mail messages from one computer to another using a client–server application architecture. An MTA implements both the client (sending) and server (receiving) portions of SMTP.

Management ports The management ports on the switch allows multiple simultaneous Telnet or SNMP network management sessions. Because there is no separate management port on the Layer 3 switch routers, you can configure any Fast Ethernet or Gigabit Ethernet port as a management port.

Manchester encoding Line code in which each bit of data is signified by at least one voltage level transition.

maximum transmission unit (MTU) The largest IP packet size allowed to be sent out a particular interface. Ethernet interfaces default to an MTU of 1,500 because the data field of an Ethernet frame should be limited to , bytes, and the IP packet sits inside the Ethernet frame’s data field.

Media Access Control (MAC) The lower of the two sublayers of the IEEE standard for Ethernet. It is also the name of that sublayer (as defined by the IEEE 802.3 subcommittee).

media independent The networking layers whose processes are not affected by the media being used. In Ethernet, these are all the layers from the LLC sublayer of data link upward.

media sharing Media sharing sites allow you to upload your photos, videos, and audio to a website that can be accessed from anywhere in the world. You can then share that media with the world or just a select group of friends.

medium (media) Storage and transmission channels or tools used to store and deliver information or data.

mesh networking A type of networking in which each node must not only capture and disseminate its own data, but also serve as a relay for other nodes—that is, it must collaborate to propagate the data in the network.

metric A unit of measure used by routing protocol algorithms to determine the best route for traffic to use to reach a particular destination.

metro Ethernet A metropolitan area network (MAN) that is based on Ethernet standards. It is commonly used to connect subscribers to a larger service network or the Internet. Businesses can also use metro Ethernet to connect their own offices to each other.

modular switches There are two types of switches available, modular and fixed configuration. Modular switches enable you to plug in different modules, allowing for scalable performance, configuration flexibility, and incremental expansion.

multicast Sending a message to selected hosts that are part of a group. A single packet is copied by the network and sent to a specific subset of network addresses. These addresses are specified in the destination address field. Compare with broadcast and unicast.

multicast MAC address The IANA owns the OUI MAC address 01:00:5e, therefore multicast packets are delivered by using the Ethernet MAC address range 01:00:5e:00:00:00 to 01:00:5e:7f:ff:ff.

multicast transmission See multicast.

multiplexing A process where multiple digital data streams are combined into one signal.

neighbor solicitation A message defined by the IPv6 Neighbor Discovery Protocol (NDP), used to ask a neighbor to reply back with a neighbor advertisement, which lists the neighbor’s MAC address.

network access protocols All of the networking protocols operating over the data link and network layers of a computer network are categorized as network access protocols. These include ARP (address resolution protocol), NDP (neighbor discovery protocol), PPP (point to point protocol), tunneling protocol, and media access protocols of link layer.

network address A dotted decimal number defined by the IPv4 protocol to represent a network or subnet. It represents the network that hosts reside in. Also called a network number or network ID.

network applications Applications in which either the program you are using or the data you are working with or both reside on a network (often, but not always, the Internet). Network applications use a client-server architecture, where the client and server are two computers connected to the network. The server is programmed to provide some service to the client.

network media Network media (sometimes referred to as networked media) refers to media mainly used in computer networks.

network service A data storage, manipulation, presentation, communication, or other capability that is often implemented using a client-server or peer-to-peer architecture based on network protocols running at the application layer of a network.

network-aware applications Network services that are always listening on the network for requests. Examples are DNS and DHCP.

next hop The next gateway to which a Layer 3 packet is delivered. The next “hop” used to reach its destination.

nibble boundary A nibble is 4 bits or one hexadecimal digit. A nibble boundary is using nibble aligned for subnet masks. By borrowing bits from the interface ID, the best practice is to subnet on a nibble boundary.

nonreturn to zero (NRZ) Line code in which 1s are represented by one significant condition and 0s are represented by another.

nslookup A service or a program to look up information in the DNS (Domain Name System).

nonvolatile RAM (NVRAM) Random-access memory that does not lose its contents when the computer is shut down.

operating system features and services An operating system is the core software that allows a computer to run as an useful device, it manages the hardware, the user interface and all other software running on the computer. Each operating system available has its own set of features and services.

OSI model Open System Interconnection reference model. A network architectural model developed by the ISO. The model consists of seven layers, each of which specifies particular network functions, such as addressing, flow control, error control, encapsulation, and reliable message transfer.

outgoing interface In a routing table, part of a routing table entry that refers to the local interface out which the local router should forward packets that match the route.

out-of-band access An important tool that enables you to gain access of equipment independent of the network connections. It essentially provides you with a backup path in case of network communication failure.

overhead Resources used to manage or operate the network. Overhead consumes bandwidth and reduces the amount of application data that can be transported across the network.

packet buffer Memory space that is set aside specifically for either storing a packet that is awaiting transmission over a network or storing a packet that has been received over a network. The memory space is either located in the network interface card or in the computer that holds the card.

payload The actual data that is encapsulated in a packet. The minimum payload is 42 octets when an 802.1Q tag is present and 46 octets when absent. The maximum payload is 1,500 octets. Nonstandard jumbo frames allow for larger maximum payload size.

peer-to-peer (P2P) Calls for each network device to run both client and server portions of an application. Also describes a small local network where hosts can play the role of client and/or server.

peer-to-peer (P2P) file sharing The distribution and sharing of digital documents and computer files using the technology of peer-to-peer (P2P) networking.

peer-to-peer (P2P) networking A type of decentralized and distributed network architecture in which individual nodes in the network (called “peers”) act as both suppliers and consumers of resources, in contrast to the centralized client–server model where client nodes request access to resources provided by central servers.

personal firewalls An application that controls network traffic to and from a computer, permitting or denying communications based on a security policy. Typically it works as an application layer firewall.

physical components Refers to the physical components making up a modern organization. Components such as servers, computers, routers and switches, modems, storage devices and such.

Physical layer The Physical layer consists of the basic networking hardware transmission technologies of a network. It is a fundamental layer underlying the logical data structures of the higher level functions in a network. Due to the plethora of available hardware technologies with widely varying characteristics, this is perhaps the most complex layer in the OSI architecture.

physical topology The arrangement of the nodes in a network and the physical connections between them. This is the representation of how the media is used to connect the devices.

podcast A digital media file or files that are distributed over the Internet using syndication feeds for playback on portable media players and personal computers.

Point-to-Point Protocol (PPP) A protocol that provides router-to-router and host-to-network connections over synchronous point-to-point and asynchronous point-to-point circuits.

port number A field in a TCP or UDP header that identifies the application that either sent (source port) or should receive (destination port) the data inside the data segment.

port-based and shared memory In port-based memory buffering, frames are stored in queues that are linked to specific incoming and outgoing ports. In shared memory buffering all frames are stored into a common memory buffer that all the ports on the switch share.

POST message Power-on self-test (POST) is a process performed by firmware or software routines immediately after many digital electronic devices are powered on. Perhaps the most widely known usage pertains to computing devices (personal computers, PDAs, networking devices such as routers, switches, intrusion detection systems and other monitoring devices). Other devices include kitchen appliances, avionics, medical equipment, laboratory test equipment—all embedded devices. The routines are part of a device’s pre-boot sequence. When POST completes successfully, bootstrap loader code is invoked.

Post Office Protocol (POP) A protocol that allows a computer to retrieve email from a server.

Power-on self-test (POST) See POST message.

preamble and start frame delimiter fields A data packet on an Ethernet link is called an Ethernet frame. A frame begins with preamble and start frame delimiter. A frame starts with a 7-octet preamble and 1-octet start frame delimiter (SFD).

prefix length In IP subnetting, this refers to the portion of a set of IP addresses whose value must be identical for the addresses to be in the same subnet.

Presentation layer In the seven-layer OSI model of computer networking, the presentation layer is layer 6 and serves as the data translator for the network. It is sometimes called the syntax layer. The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. An example of a presentation service would be the conversion of an EBCDIC-coded text computer file to an ASCII-coded file.

private address Defined in RFC 1918, an IP address that does not have to be globally unique because the address exists inside packets only when the packets are inside a single private IP internetwork. Private IP addresses are popularly used in most companies today, with NAT translating the private IP addresses into globally unique IP addresses, so the IP packet can be routed on the public Internet.

privileged executive (EXEC) mode An IOS administrative-level mode that supports access to configuration and management commands.

protocol data unit (PDUs) A generic term from OSI that refers to the data, headers, and trailers about which a particular networking layer is concerned.

protocol suite A delineation of networking protocols and standards into different categories.

protocols A written specifications that defines what tasks a service or device should perform. Each protocol defines messages.

public address An IP address that has been registered with IANA or one of its member agencies, which guarantees that the address is globally unique. Globally unique public IP addresses can be used for packets sent through the Internet.

pulse-coded modulation (PCM) A method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, CDs, digital telephony, and other digital audio applications. In a PCM stream, the amplitude of the analog signal is sampled regularly at uniform intervals, and each sample is quantized to the nearest value within a range of digital steps. PCM streams have two basic properties that determine their fidelity to the original analog signal: the sampling rate, the number of times per second that samples are taken; and the bit depth, which determines the number of possible digital values that each sample can take.

PUT message Adds a new message to the back of the message queue. A visibility timeout can also be specified to make the message invisible until the visibility timeout expires. A message must be in a format that can be included in an XML request with UTF-8 encoding.

Quarantine A method of taking questionable packets and “quarantine” them so the user has an opportunity to examine the packet and either process the packet or delete the packet.

Real-Time Transport Control Protocol (RTCP) RTP is used in conjunction with the RTCP. Although RTP carries the media streams (for example, audio and video), RTCP is used to monitor transmission statistics and quality of service (QoS) and aids synchronization of multiple streams. RTP is originated and received on even port numbers and the associated RTCP communication uses the next higher odd port number.

Real-Time Transport Protocol (RTP) Defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services, and web-based push-to-talk features.

reconnaissance attacks A kind of information gathering on network system and services. This enables the attacker to discover vulnerabilities or weaknesses on the network. It could be likened to a thief surveying through a car parking lot for vulnerable—unlocked—cars to break into and steal.

remote access via SSH See Secure Shell (SSH).

remote host A computer that resides in some distant location from which data are retrieved. It typically refers to a server in a private network or the public Internet. However, it can also refer to a user’s PC in another location that is accessed over the Internet for file transfer or remote control operation.

remote routes Routes to networks that are not directly connected to a router.

Requests for Comments (RFC) Series of documents and memoranda encompassing new research, innovations, and methodologies applicable to Internet technologies. RFCs are a reference for how technologies should work.

ring network A network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node—a ring. Data travels from node to node, with each node along the way handling every packet.

Rollover cable Also known as Cisco console cable or a Yost cable. A type of null-modem cable that is often used to connect a computer terminal to a router’s console port. This cable is typically flat (and has a light blue color) to help distinguish it from other types of network cabling. It gets the name rollover because the pinouts on one end are reversed from the other, as if the wire had been rolled over and you were viewing it from the other side. This cabling system was invented to eliminate the differences in RS-232 wiring systems. Any two RS-232 systems can be directly connected by a standard rollover cable and a standard connector. For legacy equipment, an adapter is permanently attached to the legacy port.

ROM (read-only memory) A class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware (software that is very closely tied to specific hardware, and unlikely to need frequent updates).

route timestamp Timestamp is added to the routing table entry and is used to timeout routing entries.

router A network device, typically connected to a range of LAN and WAN interfaces, that forwards packets based on their destination IP addresses.

Router Advertisement message (RA) A message defined by the IPv6 Neighbor Discovery Protocol (NDP), used by routers to announce their willingness to act as an IPv6 router on a link. These may be sent in response to a previously received NDP Router Solicitation (RS) message.

Router Solicitation message (RS) A message defined by the IPv6 Neighbor Discovery Protocol (NDP), used to ask any routers on the link to reply, identifying the router, plus other configuration settings (prefixes and prefix lengths).

routing The process by which a router receives an incoming frame, discards the data-link header and trailer, makes a forwarding decision based on the destination IP address, adds a new data-link header and trailer based on the outgoing interface, and forwards the new frame out the outgoing interface.

Running-Config file In Cisco IOS switches and routers, the name of the file that resides in RAM memory, holding the device’s currently used configuration.

scalability The ability of a protocol, system, or component to be modified to fit a new need.

Secure Shell (SSH) A protocol that provides a secure remote connection to a host through a TCP application.

security Consists of the provisions and policies adopted by a network administrator to prevent and monitor unauthorized access, misuse, modification, or denial of a computer network and network-accessible resources. Network security involves the authorization of access to data in a network, which is controlled by the network administrator. Users choose or are assigned an ID and password or other authenticating information that allows them access to information and programs within their authority. Network security covers a variety of computer networks, both public and private, that are used in everyday jobs conducting transactions and communications among businesses, government agencies, and individuals. Networks can be private, such as within a company, and others which might be open to public access. Network security is involved in organizations, enterprises, and other types of institutions. It does as its title explains: It secures the network, as well as protecting and overseeing operations being done. The most common and simple way of protecting a network resource is by assigning it a unique name and a corresponding password.

segment 1. A collision domain that is a section of a LAN that is bound by bridges, routers, or switches. 2. In a LAN using a bus topology, a continuous electrical circuit that is often connected to other such segments with repeaters. 3. When used as a verb with TCP, refers to the work TCP does to accept a large piece of data from an application and break it into smaller pieces. When used as a noun with TCP, refers to one of those smaller pieces of data.

segmentation In TCP, the process of taking a large chunk of data and breaking it into small-enough pieces to fit within a TCP segment without breaking any rules about the maximum amount of data allowed in a segment.

SEQ number (sequence number) Number used to ensure correct sequencing of the arriving data.

serial WAN interfaces A type of interface on a router, used to connect to some types of WAN links, particularly leased lines and Frame Relay access links.

Server Message Block (SMB) An application-level network protocol mainly applied to shared access to files, printers, serial ports, and miscellaneous communications between nodes on a network.

server-based firewalls A firewall that is hosted on a server, providing firewall services to hosts in its network.

service provider routers Routers optimized for the use by service providers.

Session layer In the seven-layer OSI model of computer networking, the session layer is Layer 5. The session layer provides the mechanism for opening, closing, and managing a session between end-user application processes—that is, a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session-layer services are commonly used in application environments that make use of remote procedure calls (RPCs).

shielded twisted-pair (STP) cable A type of network cabling that includes twisted-pair wires, with shielding around each pair of wires, as well as another shield around all wires in the cable.

show cdp neighbors A command that shows detailed information about the Cisco devices that are directly connected to your current device, including IP addresses. CDP is a Cisco proprietary protocol and will only detect Cisco products, although there are some vendors that do work with it. Additionally, LLDP was recently released, which is an industry standard of the Cisco Discovery Protocol and is supported on newer IOS.

Show command The show all command displays most of the system configuration and status. The information displayed by this command is displayed by other show commands. Please refer to the referenced commands for specific information about the displayed information.

show ip interface brief command Displays a brief summary of the interfaces on a device. It’s useful for quickly checking the status of the device.

signaling method Ways in which a signal can be physically created: as electrical impulses that travel over copper wire, as pulses of light that travel through strands of glass or plastic, as radio transmissions that travel over the airwaves, as laser or satellite transmissions, and as infrared pulses.

Simple Mail Transfer Protocol (SMTP) An application protocol typically not used by end users. Instead, it is used by the network management software and networking devices to allow a network engineer to monitor and troubleshoot network problems.

social media Refers to the means of interactions among people in which they create, share, and exchange information and ideas in virtual communities and networks.

socket Software structure operating as a communications end point within a network device.

solicited-node multicast An IPv6 multicast address valid within the local-link (for example, an Ethernet segment or a Frame Relay cloud). Every IPv6 host will have at least one such address per interface. Solicited-node multicast addresses are used in neighbor Discovery Protocol for obtaining the Layer 2 link-layer addresses of other nodes.

source IP address The IP address of the originating host that is placed into the IP packet header.

source IP address field The field that holds the source IP address.

source MAC Address field The field that holds the source MAC address.

source routing The capability whereby the sender can specify the route a packet should take.

standards organizations Any organization whose primary activities are developing, coordinating, promulgating, revising, amending, reissuing, interpreting, or otherwise producing technical standards that are intended to address the needs of some relatively wide base of affected adopters.

star topology A network topology in which endpoints on a network are connected to a common central device by point-to-point links.

startup-config file In Cisco IOS switches and routers, the name of the file that resides in NVRAM memory, holding the device’s configuration that will be loaded into RAM as the running-config file when the device is next reloaded or powered on.

stateful A protocol, such as TCP, to track actual conversations and their state of the communication session.

Stateless Address Autoconfiguration (SLAAC) Plug-and-play IPv6 feature that enables devices to connect themselves to the network without any configuration and without any servers (like DHCP servers).

stateless protocol A communications protocol that treats each request as an independent transaction that is unrelated to any previous request so that the communication consists of independent pairs of requests and responses. A stateless protocol does not require the server to retain session information or status about each communications partner for the duration of multiple requests. In contrast, a protocol that requires keeping of the internal state on the server is known as a stateful protocol. Examples of stateless protocols include the Internet Protocol (IP), which is the foundation for the Internet, and the Hypertext Transfer Protocol (HTTP), which is the foundation of data communication for the World Wide Web.

static addressing When network uses static addressing, each network interface has an assigned IP address that it uses all of the time or whenever it is online. When a network uses dynamic addressing, when a network interface asks to join the network, it is randomly allocated an IP address from a pool of available addresses within that network. Thus, under dynamic addressing, a computer may possess over time (for example, across reboots) a variety of different IP addresses, but under static addressing the computer has a well-defined IP address that it uses always and that no other computer ever uses. Dynamic addressing is most useful in applications such as dial-up networks, VPNs, and similar scenarios where end-user machines are intermittently connected to the network.

storage area network (SAN) A dedicated network that provides access to consolidated, block-level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices to the operating system. A SAN typically has its own network of storage devices that are generally not accessible through the LAN by other devices. The cost and complexity of SANs dropped in the early 2000s to levels allowing wider adoption across both enterprise and small to medium-sized business environments.

store-and-forward switching A method of internal processing by LAN switches. The switch must receive the entire frame before it sends the first bit of the frame. Store-and-forward switching is the method used by Cisco switches.

subnet A group of IP addresses that have the same value in the first part of the IP addresses, for the purpose of allowing routing to identify the group by that initial part of the addresses. IP addresses in the same subnet typically sit on the same network medium and are not separated from each other by any routers. IP addresses on different subnets are typically separated from one another by at least one router. Subnet is short for subnetwork.

subnet mask A dotted decimal number that helps identify the structure of IP addresses. The mask represents the network and subnet parts of related IP addresses with binary 1s and the host part of related IP addresses with binary 0s.

Switch Form-Factor Pluggable (SFP) Removal modules used in routers and switches to support a number of different network media.

SYN A 1-bit flag in the TCP header used to indicate the initial value of the sequence number. The SYN flag is only set in the first two segments of the three-way TCP connection establishment sequence.

SYN segment See three-way handshake.

synchronous Communication that uses a common clocking signal. In most synchronous communication, one of the communicating devices generates a clock signal into the circuit. Additional timing information is not required in the header.

TCP/IP (Transmission Control Protocol/Internet Protocol) A network model defined by the IETF that has been implemented on most computers and network devices in the world.

telecommunication rooms A room that consolidates all connectivity from the enterprise network and building control systems and distributes it to predetermined areas of the enterprise. It is the “horizontal” convergence point between the building’s network backbone and workstations and conference rooms.

Telnet Network service that supports CLI access to a remote host. It also can be used verify the application layer software between source and destination stations.

test-net address The IPv4 address block 192.0.2.0 to 192.0.2.255 (192.0.2.0 /24) that is set aside for teaching and learning purposes. These addresses can be used in documentation and network examples.

Trivial File Transfer Protocol (TFTP) Simplified version of FTP that allows files to be transferred from one computer to another over a network.

three-way handshake To establish a connection, TCP uses a three-way handshake. Before a client attempts to connect with a server, the server must first bind to and listen at a port to open it up for connections—this is called a passive open. When the passive open is established, a client may initiate an active open. To establish a connection, the three-way (or 3-step) handshake occurs:

1. SYN: The active open is performed by the client sending a SYN to the server. The client sets the segment’s sequence number to a random value A.

2. SYN-ACK: In response, the server replies with a SYN-ACK. The acknowledgment number is set to one more than the received sequence number—that is, A+1—and the sequence number that the server chooses for the packet is another random number, B.

3. ACK: Finally, the client sends an ACK back to the server. The sequence number is set to the received acknowledgement value—that is, A+1—and the acknowledgement number is set to one more than the received sequence number—that is, B+1.

At this point, both the client and server have received an acknowledgment of the connection. The steps 1, 2 establish the connection parameter (sequence number) for one direction and it is acknowledged. The steps 2, 3 establish the connection parameter (sequence number) for the other direction and it is acknowledged. With these, a full-duplex communication is established.

Time-to-Live (TTL) field Time to live (TTL) or hop limit is a mechanism that limits the lifespan or lifetime of data in a computer or network. TTL may be implemented as a counter or timestamp attached to or embedded in the data. After the prescribed event count or time span has elapsed, data is discarded. In computer networking, TTL prevents a data packet from circulating indefinitely. In computing applications, TTL is used to improve performance of caching or to improve privacy.

topology The arrangement of networking components or nodes. Examples include star, extended star, ring, and mesh.

traffic class field The bits of this field in the IPv6 packet hold two values. The 6 most-significant bits are used for differentiated services, which is used to classify packets. The remaining two bits are used for ECN; priority values subdivide into ranges: traffic where the source provides congestion control and noncongestion control traffic.

trailer The trailer indicates the frame check sequence number. It is used for error control in a frame.

translation In computer networking, Network Address Translation (NAT) is the process of modifying IP address information in IPv4 headers while in transit across a traffic routing device.

Transmission Control Protocol (TCP) A Layer 4 protocol of the TCP/IP model, TCP lets applications guarantee delivery of data across a network.

Transmission Control Protocol/IP (TCP/IP) TCP/IP (Transmission Control Protocol/Internet Protocol) is the basic communication language or protocol of the Internet. It can also be used as a communications protocol in a private network (either an intranet or an extranet). When you are set up with direct access to the Internet, your computer is provided with a copy of the TCP/IP program just as every other computer that you may send messages to or get information from also has a copy of TCP/IP.

tunneling Computer networks use a tunneling protocol when one network protocol (the delivery protocol) encapsulates a different payload protocol. By using tunneling one can (for example) carry a payload over an incompatible delivery-network, or provide a secure path through an untrusted network. Tunneling typically contrasts with a layered protocol model such as those of OSI or TCP/IP. The delivery protocol usually (but not always) operates at a higher level in the model than does the payload protocol, or at the same level.

Unicast MAC address A MAC address in networking that represents a single device or interface, instead of a group of addresses (as would be represented by a multicast or broadcast address).

unshielded twisted-pair (UTP) cable A general type of cable, with the cable holding twisted pairs of copper wires and the cable itself having little shielding.

User Datagram Protocol (UDP) Connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery.

user executive (EXEC) mode The limited CLI mode where the commands available to the user are a subset of those available at the privileged level. In general, use the user EXEC commands to temporarily change terminal settings, perform basic tests, and list system information.

version field The first header field in an IP packet is the four-bit version field. For IPv4, this has a value of 4 (hence the name IPv4).

virtual interface A virtual interface or virtual network interface (VIF) is an abstract virtualized representation of a computer network interface that may or may not correspond directly to a physical network interface.

Voice over IP (VoIP) Voice data encapsulated in an IP packet that allows it to traverse already implemented IP networks without needing its own network infrastructure.

VTY password Abbreviated as VTY, virtual teletype is a command-line interface that enables users to connect to the daemon using the Telnet protocol. To connect to a VTY users must set up and use a VTY password.

weblogs (blogs) A personal website or web page on which an individual records opinions, links to other sites, etc. on a regular basis.

Wide Area Network (WAN) A part of a larger network that implements mostly OSI Layer 1 and 2 technology, connects sites that typically sit far apart, and use a business model in which a consumer (individual or business) must lease the WAN from a service provider (often a Telco).

wiki A website that lets visitors add, edit, and delete content, typically without the need for registration. A good example of this is the site Wikipedia.com, where visitors can access the website and add their commentaries to already written articles or create a new article.

window size As filed in the TCP header that is set in a sent segment, signifies the maximum amount of unacknowledged data the host is willing to receive before the other sending host must wait for an acknowledgment. Used for flow control.

wireless See wireless technology.

wireless access point (WAP) A network device that provides connectivity of wireless clients to connect to a data network.

Wireless LAN (WLAN) A Local Area Network (LAN) that physically transmits bits using radio waves. The name “wireless” compares these LANs to more traditional “wired” LANs, which are LANs that use cables (which often have copper wires inside).

wireless technology Technology that allows communication without needing physical connectivity. Examples of wireless technology include cellular telephones, personal digital assistants (PDA), wireless access points, and wireless NICs.