When a Win2K or later client that already has a DHCP lease tries to boot, it attempts to renew its lease with its previous DHCP server by sending a DHCPRequest packet. If the DHCP server responds with a DHCPAck packet, the client renews its lease. If the DHCP server responds with a DHCPNack, the client restarts the lease process.

 If the DHCP server doesn't respond, the client pings the default gateway defined in the current lease. If the Ping succeeds, the client continues to use its current lease, attempting to renew at 50 percent of its assigned lease time. If the Ping fails, the client autoconfigures the IP address and continues to attempt to find a DHCP server in the background.

You can configure clients to release leases on shutdown by performing this registry change:

1. Start the registry editor (regedit.exe).
2. Navigate to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{<Interface ID} registry subkey.
3. From the Edit menu, select New, DWORD Value.
4. Enter a name of ReleaseOnShutDown and press Enter.
5. Double-click the new value and set it to 2 to release the lease according to the DHCP server's instructions (which is the default) or 0 to configure the client to not release the lease on shutdown. Click OK.

To configure the DHCP server to instruct clients to release their lease when they shut down, perform these steps:

1. Start the Microsoft Management Console (MMC) DHCP snap-in (Start, Programs, Administrative Tools, DHCP).
2. Expand the DHCP server.
3. Open the scope whose options you wish to modify. Select Scope Options, and click the Advanced tab.
4. Select Microsoft Options from the Vendor class drop-down menu and select Default User Class from the User class drop-down menu. Under Available Options, select the "002 Microsoft Release DHCP Lease on Shutdown Options" check box. Set its value to one of these options:
   1 = DHCP clients send a DHCPRelease message on proper shutdown, which means they'll give up their lease.
   0 = DHCP clients don't send a DHCPRelease message on proper shutdown, which means that when the clients restart they'll attempt to renew their existing lease.