My CISCO Journal

One of the many services that we mostly need in our network is DHCP. The Dynamic Host Configuration Protocol is used to dynamically assigned IP addresses and Default gateway information to the host on a network. In simple words, the DHCP functionality can be expressed as:

As can be seen, our host is acting as a DHCP client on the LAN, it will send DHCP Discover broadcast on the network, and all the relevant data will be provided like IP Address, Subnet Mask, Default Router, DNS Server, to the requesting host. DHCP discovery is a 4 leg process:

In the above network, we have enabled Routers0 interface as our DHCP and DNS server. The client are not configured with static IP, instead DHCP option is selected on each host, to get all configuration data from the DHCP server. Once the router is up and Clients are active, the client instantly updates their IP addresses and Default gateways by sending DHCP requests. We have configured a DHCP pool by name of LOCAL_LAN pool on our router; the IP addresses assigned to the hosts are leased from this IP pool. We have also mentioned some of the address which should not be assigned by the DHCP server, using

ip dhcp excluded-address , command as these addresses are already assigned or kept for future use.

The DHCP configurations done on R0 are:

The default router and dns server address is our routers Fast Ethernet 0/0 interface IP address. We have excluded the DHCP server IP and DNS IP from the list of IPs that is going to be assigned by DHCP server. To define router as proxy DHCP server, we use ip helper command under interface config mode, this just relay all DHCP requests to the mentioned DHCP server. Last thing, in dhcp options, we can also define lease time, the time duration till which the address is valid. The address will be renewed after the lease time is elapsed.