Network Administration: Static IP Addresses

In some cases, you may want to assign a particular IP address to a particular host. One way to do this is to configure the host with a static IP address so that the host doesn’t use Dynamic Host Configuration Protocol (DHCP) to obtain its IP configuration. However, here are two major disadvantages to that approach:

  • TCP/IP configuration supplies more than just the IP address. If you use static configuration, you must manually specify the subnet mask, the Default Gateway address, the DNS server address, and other configuration information required by the host. If this information changes, you have to change it not only at the DHCP server, but also at each host that you configured statically.

  • You must remember to exclude the static IP address from the DHCP server’s scope. Otherwise, the DHCP server won’t know about the static address and may assign it to another host. Then, you’ll have two hosts with the same address on your network.

A better way to assign a fixed IP address to a particular host is to create a DHCP reservation. A reservation simply indicates that whenever a particular host requests an IP address from the DHCP server, the server should provide it the address that you specify in the reservation.

The host won’t receive the IP address until the host requests it from the DHCP server, but whenever the host does request IP configuration, it will always receive the same address.

To create a reservation, you associate the IP address that you want assigned to the host with the host’s MAC address. As a result, you need to get the MAC address from the host before you create the reservation.

You can get the MAC address by running the command ipconfig /all from a command prompt. (If that fails because TCP/IP has not yet been configured on the computer, you can also get the MAC address [the number that uniquely identifies the hardware device] by running the System Information command, which is Start→All Programs→Accessories→System Tools→System Information.)

If you set up more than one DHCP server, each should be configured to serve a different range of IP addresses. Otherwise, the servers might assign the same address to two different hosts.