Dynamic Host Configuration Protocol
(DHCP) provides the configuration parameters to
Internet hosts. DHCP does the following:
DHCP is built on a client/server model, where designated
DHCP server hosts allocate network addresses and deliver
configuration parameters to dynamically configured
By default, DHCP supports the following mechanisms for IP address
Automatic allocation—- DHCP assigns a permanent IP address to a client.
Dynamic allocation—DHCP assigns an IP address to a client for a limited period of time (or until the client explicitly relinquishes the address).
Manual allocation—The network administrator assigns an IP address to a client and DHCP is used to convey the assigned address to the client.
The format of DHCP messages is based on the format of Bootstrap Protocol (BOOTP) messages. This format supports BOOTP relay agent functionality and interoperability between BOOTP clients and DHCP servers. With BOOTP relay agents, you do not need to deploy a DHCP server on each physical network segment.
DHCP uses the two ports assigned by IANA for BOOTP. The destination UDP port 67 sends data to the server, and UDP port 68 sends data to the client.
DHCP operations are categorized into four basic phases:
IP Lease Offer
IP Lease Acknowledgement
The DHCP operations phases are often abbreviated as DORA (Discovery, Offer, Request, and Acknowledgement).
The following figure shows the basic steps that occur when a DHCP client requests an IP address from a DHCP server. The client, Host A, sends a DHCPDISCOVER broadcast message to locate a Cisco IOS DHCP server. A DHCP server offers configuration parameters (such as an IP address, a MAC address, a domain name, and a lease for the IP address) to the client in a DHCPOFFER unicast message.
Figure 1. DHCP Request for an IP Address from a DHCP Server
The client returns a formal request for the offered IP address to the DHCP server in a DHCPREQUEST broadcast message. The DHCP server confirms that the IP address has been allocated to the client by returning a DHCPACK unicast message to the client.