A client SHOULD use DHCP to reacquire or verify its IP address and network parameters whenever the local network parameters may have changed; e.g., at system boot time or after a disconnection from the local network, as the local network configuration may change without the client's or user's knowledge.
If a client has knowledge of a previous network address and is unable to contact a local DHCP server, the client may continue to use the previous network address until the lease for that address expires. If the lease expires before the client can contact a DHCP server, the client must immediately discontinue use of the previous network address and may inform local users of the problem.