DHCP excluding address on a Cisco IOS DHCP server
When implementing DHCP on a Cisco IOS device, you use the excluded-address keyword to specify which addresses or address ranges should be excluded from being assigned to DHCP clients.
This is achieved using the following command:
DHCP(config)#ip dhcp excluded-address 192.168.12.100
or
DHCP(config)#ip dhcp excluded-address 192.168.12.100 192.168.12.109
The first case excludes one address while the second case excludes a range of addresses between the two addresses indicated, inclusive.
Note also that any IP address that is assigned to an interface of the local IOS router running the DHCP server will automatically be excluded if that address falls within the configured DHCP scopes.