Figure 18-2 DHCP State Machine
Server Discovery
When a client first boots up and comes online on the
network, it broadcasts a DHCPDISCOVER message with
a destination address of the all-subnets broadcast
address (255.255.255.255) with a source address of
0.0.0.0 since the client doesn’t have any IP address at
this stage. If there is a DHCP server configured for this
subnet, it receives the all-subnets broadcast message and
responds with a DHCPOFFER message containing the
network configuration parameters for the client. If there
isn’t a DHCP server directly configured on the same
subnet as the clients but there is a DHCP relay agent, the
agent forwards the request to the DHCP server and will
also forward the server offer to the requesting client.
Lease Offer
A DHCP server that receives a DHCPDISCOVER
message from a client responds on UDP port 68 with a
DHCPOFFER message addressed to that client. The
DHCPOFFER message contains initial network
configuration information for the client. There are
several fields in the DHCPOFFER message that are of
interest for the client: