In some older routers you have to enable the dhcp service with the above command
R1(config)#ip dhcp pool <pool name> (pool name can be any name, a pool is a range of IP addresses that the DHCP server can assign)
R1(dhcp-config)#network 192.168.100.0 255.255.255.0 (here you will specify the network that you are going to use with the subnet masks. In some router one can also use the CIDR notation for the mask like /24 )
R1(dhcp-config)#default-router 192.168.100.1 (this is the default gateway for the dhcp clients)
R1(dhcp-config)#lease 1 (the allocated address will be leased for one day)
R1(config)# ip dhcp excluded-address 192.168.100.1 1918.104.22.168 (these are the excluded address to avoid address conflict in the network)
R1# sh ip dhcp binding (A binding is an IP address that is associated with a MAC address. Each time the specified host requests an IP address, the DHCP server will assign it the address specified in the binding.
Note: This is a very basic DHCP configuration. There are some other advance DHCP options as well.