DHCP is a protocol used to automatically assign IP addresses to devices on a network. There are two DHCP features that I want to talk about today which are “release” and “renew.”
Release- When a client sends a DHCP release, the host is prematurely relinquishing its IP address lease. When IP address leases are assigned there is a lease duration and when that duration time is met then the IP address is released back into the IP address pool. A release request forfeits the IP address lease before the lease duration times out, allowing the IP address to immediately return to the pool instead of waiting the full duration.
Renew- A DHCP “renew” request is a feature that is automatically done within the DHCP protocol. During the IP address lease, a host will request the DHCP server to extend the lease to be able to use the same IP address longer. The DHCP renew is sent in a DHCP request message sent as a unicast to the server. There are two stages where a host will automatically request for a renewal. These intervals are called “T1” and “T2.” T1 is when the lease duration has reached 50% of the time used. This will be the first request to the DHCP server where the address was leased from. If the DHCP server does not respond to the first DHCP renew request, the client will resend a DHCP request at T2 which is 87.5% of the lease duration. This DHCP request will be different and will not be sent only to the target server but will be broadcast to every DHCP server on the local network. If none of the servers respond the host loses its IP address and the host must restart the DORA process again.
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