Addressing and Delivering
As you learned in Hour 3, “The Network Access Layer,” a computer communicates with the network through a network interface device such as a network adapter card. The network interface device has a unique physical address and is designed to receive data sent to that physical address. This unique physical address (which is often called the MAC address) is burned into the card when it is manufactured. A device such as an ethernet card does not know any of the details of the upper protocol layers. It does not know its IP address or whether an incoming frame is being sent to Telnet or FTP. It just listens to incoming frames, waits for a frame addressed to its own physical address, and passes that frame up the protocol stack.
This physical addressing scheme works well on an individual LAN segment. A network that consists of only a few computers on an uninterrupted medium can function with nothing more than physical addresses. Data can pass directly from network adapter to network adapter using the low-level protocols associated with the Network Access layer.
Unfortunately, on a routed network, it is not possible to deliver data by physical address. The discovery procedures required for delivering by physical address do not work across a router interface. Even if they did work, delivery by physical address would be cumbersome because the permanent physical address built into a network card does not allow you to impose a logical structure on the address space.
TCP/IP therefore makes the physical address invisible and instead organizes the network around a logical, hierarchical addressing scheme. This logical addressing scheme is maintained by the IP protocol at the Internet layer. The logical address is called the IP address. Another Internet layer protocol called Address Resolution Protocol (ARP) assembles a table that maps IP addresses to physical addresses. This ARP table is the link between the IP address and the physical address burned into the network adapter card.
On a routed network (see Figure 4.1), the TCP/IP software uses the following strategy for sending data on the network:
If the destination address is on the same network segment as the source computer, the source computer sends the packet directly to the destination. The IP address is resolved to a physical address using ARP, and the data is directed to the destination network adapter.
If the destination address is on a different segment from the source computer, the following process begins:
The datagram is directed to a gateway. A gateway is a device on the local network segment that is capable of forwarding a datagram to other network segments. (As you learned in Hour 1, “What Is TCP/IP?” a gateway is basically a router.) The gateway address is resolved to a physical address using ARP, and the data is sent to the gateway’s network adapter.
The datagram is routed through the gateway to a higher-level network segment (refer to Figure 4.1) where the process is repeated. If the destination address is on the new segment, the data is delivered to its destination. If not, the datagram is sent to another gateway.
The datagram passes through the chain of gateways to the destination segment, where the destination IP address is mapped to a physical address using ARP and the data is directed to the destination network adapter.
Figure 4.1. The gateway receives datagrams addressed to other networks.
To deliver data on a complex routed network, the Internet layer protocols must therefore be able to
Identify any computer on the network.
Provide a means for determining when a message must be sent through the gateway.
Provide a hardware-independent means of identifying the destination network segment so that the datagram will pass efficiently through the routers to the correct segment.
Provide a means for converting the logical IP address of the destination computer to a physical address so that the data can be delivered to the network adapter of the destination computer.
The most common version of IP is IPv4, although the world is theoretically in transition to a new version of IP known as IPv6. In this hour you’ll learn about the important IPv4 addressing system, and you’ll learn how TCP/IP delivers datagrams on a complex network using the Internet layer’s IP and ARP. You’ll also learn about the Internet layer’s ICMP protocol, which provides error detection and troubleshooting. For a discussion of the alternative IPv6 address system, which may eventually be the standard for Internet communication, see Hour 13, “IPv6—The Next Generation.”
By the Way
The Internet layer corresponds to the OSI Network layer, which is sometimes called Layer 3.