DHCP stands for Dynamic Host Configuration Protocol, a protocol used to simplify the management of TCP/IP clients on a network. With DHCP, a client can automatically obtain IP addresses, subnet masks, and other TCP/IP configuration settings from a DHCP server. This is easier than the alternative—manually configuring a static IP address for every client on your network.
For a third method of configuring TCP/IP clients, see Automatic Private IP Addressing (APIPA) under TCP/IP later in this chapter.
When a DHCP client starts up, it contacts a DHCP server and asks to lease an IP address. The DHCP server responds by selecting an available IP address from a scope, a range of addresses that it manages. The server then leases the selected address to the client for a certain period of time (eight days by default), provides the client with the subnet mask associated with the address, and optionally provides the client with additional information such as a default gateway address, addresses of DNS servers, and addresses of WINS servers. Once the client has obtained a lease, the client has to renew the lease periodically with the server to maintain its current address. If the client shuts down properly, it releases its lease and the server may offer the same address to a different client unless the address has been specifically reserved for the original client.
To really understand DHCP, you need to know what’s going on with DHCP at the packet level ...