4.6. Determining the Impact of Modifying, Adding, or Removing Network Services

As we discussed in Chapter 2, there are many network services in a typical network, without which the network would not function well or may not function at all. Depending on the type of service and the type of network, removing, adding, or modifying a service could have a huge impact or none at all. You should understand the purpose of most common network services and thereby understand the impact of a change in that service.

4.6.1. Critical Information

Given a scenario, you should be able to determine the impact of modifying, adding, or removing a network service such as DHCP, DNS, or WINS. In this section, we discuss the influence that each of these services has on a network.

4.6.1.1. Dynamic Host Configuration Protocol (DHCP)

Dynamic Host Configuration Protocol (DHCP) is a service that assigns TCP/IP clients an IP address and other essential addresses when they are started on the network. Clients that are configured to obtain an address from a DHCP server will automatically broadcast a request for an IP address. All DHCP servers that hear the request will make an offer of an address to the client. Typically, the address is considered valid for a specified amount of time, referred to as a lease. Clients will attempt to renew their lease before it expires by contacting the DHCP server that gave them the address. Microsoft clients, for example, renew their leases after 50 percent of the lease time has expired. DHCP servers assign and renew these addresses based on a list of available IP addresses, referred to as a scope.

If the only DHCP server on a network is removed, the impact of the removal will be on the clients that are trying to obtain a lease from the DHCP server or trying to renew their leases. The clients that already have an IP address that is not up for renewal will not be affected at all. Likewise, clients that are set with a static address will not be affected. The clients that cannot obtain an IP address will no longer be able to communicate on the network.

If another DHCP server is available and can receive the broadcasts from the clients, it will assign the clients an IP address. Typically, broadcasts do not pass through routers, but it is possible, as defined in RFC 1542, to let DHCP server broadcasts pass through most modern routers if they are configured properly.

If a DHCP server is modified or if another DHCP server is added to a network, it is very important to ensure that the IP addresses that it is configured to assign are unique. The addresses should not duplicate any addresses on any other DHCP servers in the network or on any of the devices that are assigned static addresses. If the DHCP server duplicates address assignments, the result will be multiple IP addressing errors on the computers that have the new duplicate address and on the computers that originally had a proper IP address in the network.

Windows 2000 and Windows Server 2003 networks offer extra protection to prevent the error of duplicating addresses. Once a DHCP server is configured for a domain, any additional DHCP servers must be approved by a member of the Enterprise Admins group. The process of approving the DHCP server and allowing it to issue IP addresses on the same domain is referred to as authorization. Typically, an Enterprise Admin will not authorize a new DHCP server with-out ensuring that no duplicate address assignments exist.

4.6.1.2. Domain Name Systems (DNS)

As its name implies Domain Name System (DNS) provides name resolution on a network. Specifically, DNS provides hostname-to-IP address resolution, and vice versa. Queries that are used to resolve a hostname to an IP address are referred to as forward lookup queries. Queries that are used to resolve a known IP address to a hostname are referred to as reverse lookup queries. Often a DNS server does not work alone but instead refers queries to other DNS servers, a process called forwarding. Also, many networks have multiple DNS servers that share the same databases used to resolve queries. These shared databases are referred to as zone database files. These can be shared by DNS servers through a process called zone transfer or by attaching the information to other transfers, such as Active Directory with Windows 2000 and Windows Server 2003 servers.

If a DNS server is removed, added, or modified, the impact of the change will be completely determined by the other DNS servers that are available in the network and the configuration of the clients that use them. Zone transfers provide not only load balancing of client queries but also fault tolerance if a DNS server were to fail, or in this case be removed. Likewise, clients can be configured with multiple DNS server addresses so that if one DNS server is no longer available, they can quickly query another server. Because of these factors, the removal, addition, or modification of a DNS server could have a major impact or no impact at all on a network and its clients. On the other hand, the removal of the entire DNS service would have an impact on clients because they either would not be able to resolve hostnames to IP addresses or the process would take much longer. In addition, Windows 2000 and Windows Server 2003 domains (which use Active Directory) require the presence of a DNS server.

4.6.2. Windows Internet Name Services (WINS)

Windows Internet Name Services (WINS) is a name-resolution service that provides NetBIOS name-to-IP address resolution on a local area network. Clients that are configured with a WINS server address (either manually or by a DHCP server) will register their NetBIOS name and their IP address with the WINS server when they come onto the network. Clients that need to resolve a NetBIOS name to an IP address can use the WINS server they are configured to use. In addition, clients can be configured to use multiple WINS servers.

When a network uses multiple WINS servers, it's important that the WINS servers share the information about the clients that they have registered on the network. This sharing process is referred to as replication. Administrators can configure WINS servers to replicate at set times, after a set number of changes, or both.

When we take all of these factors into consideration, it's easier to understand that the addition, removal, or modification of a single WINS server may or may not have a tremendous impact on the network. On the other hand, the removal of the entire WINS service will have an impact on legacy clients and legacy applications that use NetBIOS name resolution. These clients may not be able to communicate effectively with the servers that would otherwise provide resources for them.

4.6.3. Exam Essentials

Explain the impact of modifying, adding, or removing a DHCP server. DHCP servers assign IP addresses and other essential network information, such as name resolution server addresses. The modification, addition, or removal of a DHCP server on a network may not have an imme-diate impact if clients have already obtained an IP address lease, but that it will have an impact when the clients attempt to renew their leases. Multiple DHCP servers can be installed in a net-work, as long as the address assignments in their scopes are unique.

Describe the impact of modifying, adding, or removing DNS services. DNS servers provide hostname-to-IP address resolution, and vice versa, on a network. DNS servers rarely work alone, but instead work together with other DNS servers to resolve queries. Clients can be configured with the addresses of multiple DNS servers for fault tolerance in regard to hostname resolution. The removal of DNS services from a network would cause a serious problem in resolving hostnames to IP addresses quickly. In fact, newer networks such as Windows 2000 and Windows Server 2003 require DNS in order for Active Directory to operate at all. The removal, addition, or modification of one DNS server (of many) may have no impact on the network.

Know the impact of modifying, adding, or removing WINS. WINS resolves NetBIOS names to IP addresses. Microsoft clients can be configured with the addresses of up to 12 WINS servers for WINS fault tolerance. Modifying, adding, or removing one WINS server might have little impact on the network, but removing the entire WINS service from the network would affect primarily legacy clients and legacy applications that use NetBIOS names.