Dynamic Host Configuration Protocol (DHCP)

DHCP is the automatic IP-address giver. In essence, an administrator configures a DHCP server with a pool of addresses to lease out to hosts. The administrator may configure the server to lease out addresses to different networks by creating several pools of addresses. DHCP server a host running a service to lease IP addresses to other hosts. The DHCP server is configured by an administrator with the pool, or scope, of addresses to be leased.

Each pool of addresses contains the information that a TCP/IP host needs to build a TCP/IP stack. The pool is sometimes referred to as a scope, or range, of addresses. As a DHCP client is building its TCP/IP stack, it requests an IP address, and the DHCP server looks in the pools of addresses to see whether there are any IP addresses that can be leased out to the network that the host is on.

Scope A pool of IP addresses that a DHCP server leases to DHCP clients. The client doesn't already have an IP address because the administrator did not type one in. Instead, the administrator determined that the client should get its IP address automatically from a DHCP server. So the client broadcasts a packet out onto the network; the packet says something like, "Hey! Who's the DHCP server?"

After a DHCP server receives a packet indicating that a DHCP client is looking for a DHCP server, the server examines its pools of addresses and possibly offers an IP address to the client. If the DHCP server is not configured with a pool of addresses for the network where the DHCP client is located, the request is ignored.

The DHCP client might receive offers from several DHCP servers. The client selects the first one that it receives and sends back a packet that says, "Yeah, I would like that IP address." That packet goes back to the DHCP server, and the DHCP server responds with a packet that says, "All right, you can have that IP address." In the following sections, you will examine the four-step process that a DHCP client goes through to obtain an IP address from a DHCP server.

Obtaining an IP Address from a DHCP Server
There are four steps in getting an IP address from a DHCP server: DHCP discover DHCP offer, DHCP request, and DHCP acknowledgment. Let's walk through these steps and examine the packets at each.

DHCP Discover

The first step is the broadcast of a DHCP discover packet. As the client is building its TCP/IP stack, it broadcasts a message that says, "Hey! I need a DHCP server!" The client broadcasts this packet so that no configuration is necessary. No administrator needs to type in the address of the DHCP server. Also, the client that is looking for a DHCP server has to broadcast the discover packet so that all hosts on the network will receive it-one of which is hopefully a DHCP server.

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