DHCP Lease Renewal

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DHCP Lease Renewal

The DHCP client attempts to renew its IP address lease after 50 percent of its lease time has expired (or when manually requested to renew the lease by the ipconfig/renew command from a command prompt). To renew the lease, a DHCP client sends a DHCPREQUEST directly to the DHCP server that gave it the original lease. Again, the DHCPREQUEST contains the hardware address of the client and the requested IP address, but this time uses the DHCP server IP address for the destination and the DHCP client IP address for the source IP address in the datagram.

If the DHCP server is available and the requested IP address is still available (has not been removed from the scope), the DHCP server responds by sending a DHCPACK directly to the DHCP client. If the server is available but the requested IP address is no longer in the configured scopes, a DHCPNACK is sent to the DHCP client, which then must start the lease process over with a DHCPDISCOVER. A DHCPNACK can be sent because of the following reasons.

The IP address requested is no longer available because the lease has been manually expired on the server and has been given to another client.
The IP address requested has been removed from the available scopes on the DHCP server.
The DHCP client has been physically moved to another subnet that will use a different scope on the DHCP server for that subnet. Hence, the IP address changes to a valid IP address for the new subnet. If the server does not respond to the DHCPREQUEST sent after the lease is 50 percent expired, the DHCP client continues to use the original lease until it is seven-eighths expired (87.5 percent of the lease time has expired). Because this DHCPREQUEST is broadcast rather than directed to a particular DHCP server, any DHCP server can respond with a DHCPACK or DHCPNACK to renew or deny the lease.

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