Network Address Assignments (Allocating a New Address)

To receive configuration information and a network address assignment, a DHCP client negotiates with DHCP servers in a series of messages. The following steps show the messages exchanged between a DHCP client and servers to allocate a new network address. When allocating a new network address, the DHCP process can involve more than one server, but only one server is selected by the client.

1. When a client computer is started, it broadcasts a DHCPDISCOVER message on the local subnet, requesting a DHCP server. This request includes the hardware address of the requesting client.

   Figure 4: DHCP Discover

   

   Note: For improved operation with DHCP clients that do not strictly conform to RFC 2131, the DHCP server accepts and processes DHCPDISCOVER messages even if the overload options in the messages are not properly terminated with an end statement.

2. Each DHCP server receiving the broadcast sends a DHCPOFFER message to the client, offering an IP address for a set period of time, known as the lease period.

   Figure 5: DHCP Offer

   

3. The client receives one or more DHCPOFFER messages from one or more servers and selects one of the offers received. Normally, a client looks for the longest lease period.
4. The client broadcasts a DHCPREQUEST message indicating the client has selected an offered leased IP address and identifies the selected server.

   Figure 6: DHCP Request

   

5. Those servers not selected by the DHCPREQUEST message return the unselected IP addresses to the pool of available addresses.
6. The selected DHCP server sends a DHCPACK acknowledgment that includes configuration information such as the IP address, subnet mask, default gateway, and the lease period.

   Figure 7: DHCP ACK

   

   The information offered by the server is configurable. See Configuring a DHCP Server for more information.

7. The client receives the DHCPACK message with configuration information. The process is complete. The client is configured and has access to the network.
   • If the client receives a DHCPNAK message (for example, if the client has moved to a new subnet), the client restarts the negotiation process.
   • The client can relinquish its lease on a network address by sending a DHCPRELEASE message to the server (for example, when the client is restarted). When the server receives the DHCPRELEASE message, it marks the lease as free and the IP address becomes available again.

Figure 8: DHCP Release

Copyright © 2009, Juniper Networks, Inc. All rights reserved. Trademark Notice.