| This focus will be on the
Cisco IOS DHCP Server.
Cisco IOS Easy IP contains a full DHCP server implementation that
assigns and manages IP addresses from specified address pool(s)
within the router to DHCP hosts. The Cisco IOS DHCP Server supports
many DHCP options as defined in RFC 2132. Following are the steps
required for a client to receive an IP address via DHCP. 
Step 1. Upon powering up on the network, the DHCP-enabled client
requests an IP address from a DHCP server. This is accomplished by
broadcasting a DHCPDISCOVER message on its local physical subnet. 
Step 2. The Cisco IOS DHCP Server receives the DHCPDISCOVER
request from the SOHO host and unicasts a DHCPOFFER message as a
response. This message offers an IP address, subnet mask, address
lease period, and several other parameters, to the client. The IP
address in the DHCPOFFER message is taken from one of potentially
several user-configured DHCP address pools. Although IP address
lease times are configurable within the Cisco IOS DHCP Server, IP
addresses are issued with a default lease period of 24 hours. 
Note: The DHCP client will time out and retransmit the
DHCPDISCOVER message if the client receives no DHCPOFFER messages.
Next, the client accepts the offer from the Cisco IOS DHCP Server by
broadcasting a DHCPREQUEST message on its local physical subnet. To
acknowledge receipt of the DHCPREQUEST and begin the address lease,
the Cisco IOS DHCP Server responds with a DHCPACK message. Arrival
of the DHCPACK message enables the client to begin using the
assigned address. The combination of the client's hardware address
and assigned network address constitutes a unique identifier for the
client's lease and is used by both the client and server to identify
a lease referred to in all DHCP messages.
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