DHCP (Dynamic Host Configuration Protocol) (Linktionary term)

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DHCP (Dynamic Host Configuration Protocol)
Expanded version: contains additional text not in the book

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:

Without DHCP	With DHCP
In manual configuration, you must assign an address at every workstation on the network. Users will need to call you for an IP address since you don't want to depend on them to configure their own IP addresses.	The DHCP server automatically leases IP addresses to users when they log on. You only need to specify the scope of addresses that can be leased at the server. You are no longer burdened by calls from users who need an IP address, or worse, the need to go on-site and configure the address.
Configuring a large number of addresses may lead to errors that are difficult to track down and may cause errors in communication on the network.	DHCP automatically manages IP addresses and eliminates errors that might disrupt communication. It automatically reassigns unused addresses.
You'll eventually run out of IP addresses for a subnet of the network or for the entire network if you don't carefully manage the assigned addresses.	DHCP leases addresses for a period of time, which means that addresses are made available to assign to other systems. You are less likely to run out of available addresses.
You must change the IP address in a workstation if it moves to another subnet.	DHCP automatically assigns an IP address that is appropriate for the subnetwork to which the workstation attaches.
Mobiles users that move from one location to another will need to change the IP addresses of their computers if they connect with a different subnet of the network.	As above, DHCP automatically assigns IP addresses to mobile users at the subnet where they attach. Mobile computing becomes more of a reality as management headaches are reduced.

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:

Step 1 The client workstation boots and initializes with a "null IP address" that lets it communicate with the DHCP server using TCP/IP. It prepares a message that contains its MAC address (for example, the hardwired address of its Ethernet adapter) and its computer name. The message may also contain a previous IP address that it has leased from a DHCP server. The client "broadcasts" the message on the network and continues to send the message until it receives a response from the server.
Step 2 Any DHCP server can receive the message and prepare to lease the client an IP address. If a server has a valid configuration for the client, it prepares an "offer" message, which contains the client MAC address, the IP address that the server is offering to lease, a subnet mask, the IP address of the server, and the time length of the lease. The offered address is marked as "reserved." DHCP servers broadcast offer messages over the network.
Step 3 When the client receives the offer messages and accepts one of the IP addresses, the client broadcasts a message to confirm which DHCP server it has accepted an IP address from.
Step 4 Finally, the DHCP server confirms the whole arrangement with the client.

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.

DHCP Developments

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:

RFC 1542 (Clarifications and Extensions for the Bootstrap Protocol, October 1993)
RFC 2131 (Dynamic Host Configuration Protocol, March 1997)
RFC 2132 (DHCP Options and BOOTP Vendor Extensions, March 1997)
RFC 2241 (DHCP Options for Novell Directory Services, November 1997)
RFC 2242 (Netware/IP Domain Name and Information, November 1997)
RFC 2485 (DHCP Option for The Open Group's User Authentication Protocol, January 1999)
RFC 2489 (Procedure for Defining New DHCP Options, January 1999)
RFC 2610 (DHCP Options for Service Location Protocol, June 1999)
RFC 3074 (DHCP Load Balancing Algorithm, February 2001)