DHCP

The DHCP Process

When TCP/IP is first initialized during installation, the client is configured to use a DHCP server. The DHCP server will provide the client with an IP address and a subnet mask. DNS,WINS and gateway addresses can also be given out. The address is leased to the client for a specified amount of time or until the client shuts down. If a DHCP server is unavailable the client will use Automatic Private IP Addressing (APIPA). The client will assign itself an address between 169.254.0.0 and 169.254.254.254. All clients on the same subnet using APIPA will be able to see each other.

The Client boots up and requests an IP address. The client sends out a broadcast message on the network,known as a DHCP Discover message. The DHCP Discover packet is picked up by a DHCP Server.

Each DHCP server that receives the discover message responds with a DHCP Offer message. The message contains an IP address which the client can use.

The client computer receives the offer messages and selects one (usually the first one it receives) by sending a DHCP Request message with the IP address it wants.

The DHCP Server will then send an acknowledgement to the client, saying that the address has been leased to it. The client will then configure itself using the information.

DHCP Scopes

Once a DHCP server has been installed a DHCP scope will need to be configured. A DHCP scope contains a range of IP addresses that can be given out to clients on the network. The scope should never contain IP addresses that are already in use, e.g. The DHCP server’s own IP address or the address of a WINS/DNS server. A DHCP scope can also contain additional information such as the default gateway, DNS and WINS server addresses.

An IP address is leased to the client for a default period of eight days. The client will attempt to renew its lease well before the lease period is up. A computer will release its IP address when it is shutdown correctly, this enables the IP address to be used by another machine when not in use. The lease duration can be shortened. This is ideal for more dynamic networks, such as those which include mobile laptop users.