The client can return the address to the server at any time, but must request an extension from the server to retain the address longer than the time permitted. The server automatically reclaims the address after the lease expires if the client has not requested an extension. Dynamic allocation is useful in a large distributed network where many systems are being added and deleted. Unused addresses are returned to the pool of addresses without relying on users or system administrators to take action to return them. Addresses are only used when and where they're needed.
Dynamic allocation allows a network to make the maximum use of a limited set of addresses.
DHCP suddenly stopped working on one of my hosts. We allow you to temporarily boot a new host to the network as an unregistered host so you can access this web site and register your host for DHCP service. Registration of all DHCP clients allows us to keep track of IP address usage. It enables us to keep an eye on networks that might need extra capacity to accommodate growing user bases, diagnose problems, audit traffic, and generally help you manage your network. It's just good policy.