We recently temporarily powered down our primary domain controller which runs the DHCP service. This was part of a of a larger upgrade process. The next day, while this server was still down, all the client PCs were powered on and had valid IP address. Doing a IPConfig showed a valid lease obtained, the lease renewal date was correct future date and the DHCP server was the original DHCP server. I expect Active Directory or DNS is involved in the answer. All this with no DHCP. Clients are all Windows XP SP2 and SP3. The question is how and why did the clients retain their IP addresses?