DHCP binds the assigned IP address to the hardware address. You can move systems around in the same building and always get the same IP address. A system maintains its assigned (leased) IP address by being active on the network. Leases are good for 30 days, therefore a system would have to be off the network, or powered down for 30 days before the IP address would be revoked. This is never an issue for servers that are always active except for periodic maintenance power downs.