In my last post, I explained about the DHCP lease timer and received a lot of questions about how to renew DHCP leases. Here's how it works.
You probably noticed on your home computer that you have had the same IP address for months, perhaps longer. If you have a work computer, you may have noticed that it may have the same IP address, but it may be assigned by DHCP.
My previous post stated that most of the DHCP server has a default lease duration of 8 days. In this case, every time you lease any IP address, you will have it for 8 days. For example, if I have a desktop that is always on your network, then it makes no sense to give this desktop a new IP address every 8 days. Renewal of your existing IP address makes more sense than leasing a new one.
Imagine my DHCP server is set up and a few addresses are leased. So my client received an IP address with an 8-day lease duration, which is the default setting in my case. Here are a few facts to keep in mind.
👉 Whenever my client reaches 50% of that lease duration, which is 4 days, it will go back to the DHCP server and attempt to renew its address. It's a unicast DHCP request. The client will say to the DHCP server, Hey I already have this address assigned, You have leased it to me for 8 days, but it has already reached 50%. In this case, you might receive an ack from DHCP and your address is now renewed for another 8 days. Your lease duration will never be less than 4 days if your DHCP server is always online and your client is always online. In the event that the server rejects a renewal request, it sends a DHCP NAK message to the client so that the client can then request a new IP address using DHCP Discover.
👉 Let's say there was an issue. The renewal process fails due to some issue such as a network problem or DHCP server not being available. The DHCP lease renewal process begins again when the lease duration reaches 87.5% of the validity period, the DHCP client broadcasts a DHCP Request message to request lease renewal. I know 87.5% is an odd number :), let's do the maths. With 8 days of lease duration, you will attempt to renew on day 4, then on day 7, where this 87.5% comes into play. It's just a fact, so there is no point in remembering it. Similar to the above point, if the client receives a DHCP ACK message, the server has renewed the lease; if the client receives a DHCP NAK message, the client must send a DHCP Discover message to apply for a new IP address.
👉 The renewal intervals are not adjustable, they are what they are. If no response is received when the lease expires (when the lease reaches 100% of the lease duration ), the client stops using the IP address and sends a DHCP Discover message to apply for a new IP address.
