Understanding DHCP

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Understanding DHCP

The configuration of Microsoft TCP/IP involves you knowing the correct values for several fields for each TCP/IP host and entering them manually. At the minimum, the host IP address, and subnet mask need to be configured. In most cases other parameters, such as WINS and DNS server addresses, also need to be configured on each host. DHCP relieves the need for manual configuration and provides a method of configuring and reconfiguring all the TCP/IP related parameters.

It is critical that the correct TCP/IP address is configured on each host; otherwise, hosts on the internetwork might:

Fail to communicate
Fail to initialize
Cause other hosts on the internetwork to hang

The Dynamic Host Configuration protocol is an open industry standard that enables the automatic TCP/IP configuration of DHCP client computers. The use of Microsoft’s DHCP server greatly reduces the administrative overhead of managing TCP/IP client computers by eliminating the need to manually configure clients. The DHCP server also allows for greater flexibility and mobility of clients on a TCP/IP network without administrator intervention. If used correctly, DHCP can eliminate nearly all the problems associated with TCP/IP. The administrator enters the valid IP addresses or ranges of IP addresses (called a scope) in the DHCP server database, which then assigns (or leases) the IP addresses to the DHCP client hosts.

Having all the TCP/IP configuration parameters stored on the DHCP server provides the following benefits:

The administrator can quickly verify the IP address and other configuration parameters without having to go to each host. Also, reconfiguration of the DHCP database is accomplished at one central location, thereby eliminating the need to manually each host.
DHCP does not lease the same IP address from a scope to two hosts at the same time; this can prevent duplicate IP addresses if used properly.

DHCP cannot detect which IP addresses are already being used by non-DHCP clients. If a host has a manually configured IP address and a DHCP scope is configured with that same address, the DHCP server may lease the address to a DHCP client, creating a duplicate IP address on the network. To prevent this situation, you must exclude all manually configured IP addresses from any scopes configured on the DHCP server.
The DHCP administrator controls which IP addresses are used by which hosts. DHCP uses local network broadcasts to lease IP addresses to client hosts. If a second DHCP server resides on the same local network segment, the DHCP client can communicate with either server and may receive an IP address lease from the unintended DHCP server. See “Using Multiple DHCP Servers” later in this chapter for the ways to detect and prevent this situation.
The chance of clerical and typing errors is reduced because the TCP/IP configuration parameters are entered in one place; the DHCP server database.
Several options can be set for each DHCP scope (or globally for all scopes) that are configured on the client along with the IP address, for example, default gateway, WINS server addresses, and so on.
An IP address may be leased for a limited time, which requires the DHCP client periodically to renew its lease before the lease expires. If the host is no longer using the IP address (is no longer running TCP/IP or is powered off), the lease expires and can then be assigned to another TCP/IP host. This feature is useful if the number of hosts requesting IP addresses is larger than the number of available valid IP addresses (such as when the network is part of the Internet).
If a host is physically moved to a different subnet, the DHCP server on that subnet automatically reconfigures the host with the proper TCP/IP configuration information for that subnet.

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