Windows DHCP has a default grace period of four (4) hours. It protects a client lease in the following cases:
The client and server are in different time zones
The internal clocks of the client and server computers are not synchronized
The client is off the network when the lease expires
Please check each of the above conditions and see if that helps in DHCP releasing the address.
Check whether any devices on the network have static IP addresses that have not been excluded from the DHCP scope. Verify that the DHCP server is bound to at least one IP address, and that this is within the subnet of the scopes from which IP addresses must be leased out (unless using DHCP relay).
Hope this resolves your Query !!
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