Dynamic Host Configuration Protocol

The Dynamic Host Configuration Protocol (DHCP) is a set of rules used by a communications device (such as a computer, router or networking adapter) to allow the device to request and obtain an Internet address from a server which has a list of addresses available for assignment.

Introduction

DHCP is a protocol used by networked computers (clients) to obtain unique IP addresses, and other parameters such as default router, subnet mask, and IP addresses for DNS servers from a DHCP server. This protocol is used when computers are added to a network because these settings are necessary for the host to participate in the network. This setting is periodically refreshed (it expires, meaning the client must obtain another assignment) with typical intervals ranging from one hour to several months, and can, if desired, be set to infinite (never expire). The length of time the address is available to the device it was assigned to is called a lease, and is determined by the server.

The DHCP server ensures that all IP addresses are unique, that is, no IP address is assigned to a second client while the first client's assignment is valid (its lease has not expired). Thus IP address pool management is done by the server and not by a human network administrator.

Overview

The Dynamic Host Configuration Protocol (DHCP) automates the assignment of IP addresses, subnet masks, default routers, and other IP parameters. The assignment usually occurs when the DHCP configured machine boots up or regains connectivity to the network. The DHCP client sends out a query requesting a response from a DHCP server on the locally attached network. The query is typically initiated immediately after booting up and before the client initiates any IP based communication with other hosts. The DHCP server then replies to the client with its assigned IP address, subnet mask, DNS server and default gateway information.

The assignment of the IP address usually expires after a predetermined period of time, at which point the DHCP client and server renegotiate a new IP address from the server's predefined pool of addresses. Configuring firewall rules to accommodate access from machines who receive their IP addresses via DHCP is therefore more difficult because the remote IP address will vary from time to time. Administrators must usually allow access to the entire remote DHCP subnet for a particular TCP/UDP port.

Most home routers and firewalls are configured in the factory to be DHCP servers for a home network. An alternative to a home router is to use a computer as a DHCP server. ISPs generally use DHCP to assign clients individual IP addresses.

DHCP is a broadcast-based protocol. As with other types of broadcast traffic, it does not cross a router unless specifically configured to do so. Users who desire this capability must configure their routers to pass DHCP traffic across UDP ports 67 and 68.