2.8 Easy IP/DHCP
2.8.2 Cisco IOS DHCP server
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.