Are there anyway to release a client's ip address from server rather than in client computer typing 'ipconfig /release' ???

Why would you want to? I think I understand why you want to, but think for a second. Taking the lease out of the server doesn't make the client cease in having that IP, so if the server hands the IP address out to something else, you'd have duplicate IP's. TECH-NO-LOGICAL ROMANCE! http://www.homestarrunner.com/tgs12.html

oh ic. I understand now, very good explanation. What about renewing all the clients' ip address? Another question that i want to ask is are there any back up dhcp server? For example, if my primary dhcp server down, the secondary server will come up and become the primary. Are there such settings or softwares around?

Hi, you can do it by adding secondary DHCP server. You can refer the following link. http://www.microsoft.com/technet/pr... With regards, www.salem-radha.com

Unless you have remote administration tools installed to the client PCs you can't order them to renew their IP leases remotely, sorry. You can have the clients set to renew their leases frequently, like once every 5 minutes, and disable the IP Address the client is using just before the lease is up so that it is forced to get a new IP Address the next time it renews but that interferes with network traffic. As for your DNS question there's a bank of public DNS servers in the 4.2.2.x range, where x is between 1 and 10. They get pretty busy so only use these for your secondary DNS address.