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DHCP (Dynamic Host Configuration Protocol)
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
DHCP is designed to help reduce configuration time for TCP/IP networks by automatically assigning IP addresses to clients when they log on. DHCP centralizes IP address management on central computers that run the DHCP server program.
Although you can manually assign permanent IP addresses to any computer on your network, DHCP provides a way to automatically assign addresses. In order to have a client get its IP address from a DHCP server, you configure the client to "obtain its address automatically from a host server." This option appears in the TCP/IP configuration area of most clients' operating systems. Once these options are set, the client "leases" an IP address from the DHCP server every time it boots.
At least one DHCP server must exist on a network. Once the DHCP server software is installed, you create a DHCP scope, which is a pool of IP addresses that the server manages. When clients log on, they request an IP address from the server, and the server provides an IP address from its pool of available addresses.
DHCP was originally defined in RFC 1531 (Dynamic Host Configuration Protocol, October 1993) but the most recent update is RFC 2131 (Dynamic Host Configuration Protocol, March 1997). The IETF Dynamic Host Configuration (dhc) Working Group is chartered to produce a protocol for automated allocation, configuration, and management of IP addresses and TCP/IP protocol stack parameters.
DHCP is a boon to network administrators. It relieves configuration problems that are inherent with manual configurations. Review this chart to see how DHCP alleviates problems:
DHCP Leasing Sequence
DHCP is an Internet protocol that has its roots in the Bootstrap Protocol, or BOOTP, which is used to configure diskless workstations. DHCP takes advantage of the messaging protocol and configuration techniques that are already defined for BOOTP, including the ability to assign IP addresses. This similarity also allows existing routers that relay BOOTP messages between subnets to relay DHCP messages. Because of this, a single DHCP server can handle IP addressing for multiple subnets.
The process of acquiring an IP address is described here:
Note that clients initially broadcast IP address requests on the network, which means that any DHCP server can receive the message. Therefore, more than one DHCP server might attempt to lease the client an IP address by sending it offers. The client only accepts one offer, then broadcasts the confirmation message on the network. Since the message is broadcast, all DHCP servers can receive it. The message contains the IP address of the DHCP server that leased the IP address it will use, so other DHCP servers retract their offer to lease an IP address and return the IP address to the address pool to be assigned to other clients.
The DHCP Working Group is developing DHCP for the next generation of IP, IPv6. The group has also developed drafts that describe how DHCP autoconfiguration can automatically update the name-to-address and address-to-name mappings maintained by DNS (Domain Name System).
A number of drafts and RFCs have been written to extend DHCP in a number of ways. DHCP can integrate with LDAP (Lightweight Directory Access Protocol) directory services and NDS (Novell Directory Service). RFC 2136 (Dynamic Updates in the Domain Name System, April 1997) describes Dynamic DNS (DDNS), which defines how a DHCP server can make dynamic changes to a DNS server.
Refer to the IETF Dynamic Host Configuration (dhc) site listed on the related entries page for more information about these and other DHCP developments. Here is a list of relavent RFCs:
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.