Reference Guide

Assign an IP Address using DHCP
The following section describes DHCP and the client in a network.
When a client joins a network:
1. The client initially broadcasts a DHCPDISCOVER message on the subnet to discover available DHCP servers. This
message includes the parameters that the client requires and might include suggested values for those
parameters.
2. Servers unicast or broadcast a DHCPOFFER message in response to the DHCPDISCOVER that offers to the client
values for the requested parameters. Multiple servers might respond to a single DHCPDISCOVER; the client might
wait a period of time and then act on the most preferred offer.
3. The client broadcasts a DHCPREQUEST message in response to the offer, requesting the offered values.
4. After receiving a DHCPREQUEST, the server binds the clients’ unique identifier (the hardware address plus IP
address) to the accepted configuration parameters and stores the data in a database called a binding table. The
server then broadcasts a DHCPACK message, which signals to the client that it may begin using the assigned
parameters.
5. When the client leaves the network, or the lease time expires, returns its IP address to the server in a
DHCPRELEASE message.
There are additional messages that are used in case the DHCP negotiation deviates from the process previously
described and shown in the illustration below.
DHCPDECLINE A client sends this message to the server in response to a DHCPACK if the configuration
parameters are unacceptable; for example, if the offered address is already in use. In this
case, the client starts the configuration process over by sending a DHCPDISCOVER.
DHCPINFORM A client uses this message to request configuration parameters when it assigned an IP
address manually rather than with DHCP. The server responds by unicast.
DHCPNAK A server sends this message to the client if it is not able to fulfill a DHCPREQUEST; for example,
if the requested address is already in use. In this case, the client starts the configuration
process over by sending a DHCPDISCOVER.
Figure 37. Client and Server Messaging
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