RFC 2131

RFC 2131 is the introduction of Dynamic Host Configuration Protocol, or DHCP, in its current incarnation. This document was published in March 1997. This formalized standard is used in describing how IP, or Internet Protocol, addresses are assigned in a dynamic form. This is accomplished by using the Client-Server model of networking. This document provides an overview of the purpose and then drills down into the specifications of how Dynamic Host Configuration Protocol functions. The following is a brief review of this document.

DHCP can use three different methods assigning addresses. These are automatic allocation, dynamic allocation, and manual allocation. The most common mechanism that is used is dynamic allocation. This function assigns the IP address to a node temporally. This temporary assignment is called a lease. The lease is set for a configurable amount of time by the administrator, usually 12 hours. Once the lease time is expired, the client needs to request a new IP address. For the sake of ease, the DHCP server will usually re-assign the same address to the client.

The process that the clients and DHCP server go through for address assignment has basically six steps. In the first step, the client sends out a broadcast of a DHCPDISCOVER message. This broadcast is received by one or more DHCP servers. In the second step of the process, the DHCP servers that received the initial DHCPDISCOVER message will respond with a DHCPOFFER message to the client. This leads to the third step, the DHCPREQUEST message that is sent by the client. This is a direct response to the offer with the client using the IP address within the packet response. If the IP address has not been assigned by the time the DHCPREQUEST has been returned, the server will respond to the request with a DHCPACK message. Otherwise, the server will send a DHCPNAK message and the client will have to start the process over. If the DHCPACK message is sent in step four, the client will use the parameters contained in the packet to configure itself. The sixth and final step in the process is the process is the DHCPRELEASE message that is sent by the client to resign control of that IP address.

The IP address that is assigned during this process is kept by the client only for the period of the lease time. This time, typically 12 hours, can be configured to meet the needs of the network. Using the example of the 12 hour lease, a lease renewal will be sent by the server at half the lease time. In this example, the message will be sent at six hours remaining. This is sent to the client as another DHCPACK message. If accepted by the client, the lease is renewed. This is repeated until either the lease expires or the DHCPRELEASE message is sent by the client.
On a side note to the DHCP process, the address on a Microsoft Windows’ client can be released and renewed manually. This is done by using the commands IPCONFIG /release and IPCONFIG /renew in the command prompt window.

Droms, R. (March 1997). "DYNAMIC HOST CONFIGURATION PROTOCOL." Retrieved August 19, 2009, from http://www.ietf.org/rfc/rfc2131.txt?number=2131