What is DHCP?

Dynamic Host Configuration Protocol (DHCP) is a network protocol that enables a DHCP server to automatically assign an IP address to an individual computer's TCP/IP stack software. DHCP assigns a number dynamically from a defined range of numbers (i.e., a scope) configured for a given network.

Client computers configured to use DHCP for IP assignment do not need to have a statically assigned IP address. In addition, they generally do not need to have addresses configured for DNS servers or WINS servers, as these are also set by the DHCP server.

DHCP assigns a TCP/IP address when a system is started. Typically, it works like this:

1. A user turns on a computer with a DHCP client.

2. The client computer sends a broadcast request (called a DISCOVER or DHCPDISCOVER), looking for a DHCP server to answer.

3. The router directs the DISCOVER packet to the correct DHCP server.

4. The server receives the DISCOVER packet. Based on availability and usage policies set on the server, the server determines an appropriate address (if any) to give to the client. The server then temporarily reserves that address for the client and sends back to the client an OFFER (or DHCPOFFER) packet, with that address information. The server also configures the client's DNS servers, WINS servers, NTP servers, and sometimes other services as well.

5. The client sends a REQUEST (or DHCPREQUEST) packet, letting the server know that it intends to use the address.

6. The server sends an ACK (or DHCPACK) packet, confirming that the client has a been given a lease on the address for a server-specified period of time.

When a computer uses a static IP address, it means that the computer is manually configured to use a specific IP address. One problem with static assignment, which can result from user error or inattention to detail, occurs when two computers are configured with the same IP address. This creates a conflict that results in loss of service. Using DHCP to dynamically assign IP addresses minimizes these conflicts.