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Option Number and Description
Identifiers a user-defined string used by the Relay Agent to forward DHCP client
packets to a specific server.
L2 DHCP
Snooping
Option 82
Specifies IP addresses for DHCP messages received from the client that are to be
monitored to build a DHCP snooping database.
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 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.
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Dynamic Host Configuration Protocol (DHCP)