Restarting a DHCP Client
When a client that previously leased an IP address restarts, it broadcasts a DHCPRequest message instead of a DHCPDiscover message. The DHCPRequest message contains a request for the previously assigned IP address.
If the requested IP address can be used by the client, the DHCP server responds with a DHCPAck message.
If the IP address cannot be used by the client because it is no longer valid, is now used by another client, or is invalid because the client has been physically moved to a different subnet, the DHCP server responds with a DHCPNak message. If this occurs, the client restarts the lease process.
If the client fails to locate a DHCP server during the renewal process, it attempts to ping the default gateway listed in the current lease, with the following results:
If a ping of the default gateway succeeds, the DHCP client assumes it is still located on the same network where it obtained its current lease, and the client continues to use the current lease. By default, the client attempts, in the background, to renew its current lease when 50 percent of its assigned lease time has expired.
If a ping of the default gateway fails, the DHCP client assumes that it has been moved to a different network, where DHCP services are not available (such as a home network). By default, the client auto-configures its IP address as described previously, and continues (every five minutes in the background) trying to locate a DHCP server and obtain a lease.