Administrator Guide

Option Number and Description
User Port Stacking Option 230
Set the stacking option variable to provide DHCP server stack-port detail when the DHCP oer is set.
End Option 255
Signals the last option in the DHCP packet.
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 oers 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 oer.
3 The client broadcasts a DHCPREQUEST message in response to the oer, requesting the oered values.
4 After receiving a DHCPREQUEST, the server binds the clients’ unique identier (the hardware address plus IP address) to the
accepted conguration 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 conguration parameters are
unacceptable; for example, if the oered address is already in use. In this case, the client starts the conguration
process over by sending a DHCPDISCOVER.
DHCPINFORM A client uses this message to request conguration 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 fulll a DHCPREQUEST; for example, if the requested
address is already in use. In this case, the client starts the conguration process over by sending a
DHCPDISCOVER.
Figure 33. Client and Server Messaging
Dynamic Host
Conguration Protocol (DHCP) 291