DHCP Leases – CompTIA Network+ N10-006 – 1.3

| March 22, 2015


The DHCP lease process ensures that all devices on the network always have an IP address. In this video, you’ll learn about the DHCP lease process and the lease timers.

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When our computer receives an automatic IP configuration using DHCP, it’s really only using this IP address temporarily. It seems like it’s permanent. In fact, we always seem to have the same IP address even when we leave the office and come back after a long period of time. We also seem to have the same IP. But the reality is that this IP address is one that we constantly have to check back with to ensure that we keep that IP address on our computer.

This process of obtaining an IP address for the first time is something called allocation. The DHCP server allocates you an IP address. And it also tells you a particular lease timer that’s going to be used. This lease timer can be configured by the DHCP administrator. And this is going to determine how often our workstation checks in so that we can keep the exact same IP configuration.

If your computer forgets its IP address for some reason, then it goes through a process called reallocation. If you were to reboot your computer, for example, it forgets everything it knows about IP configuration. Then when it starts back up, it checks in with the DHCP server. And the DHCP server reallocates the same IP address information. And it also confirms and restarts that lease timer.

The workstation could also be set and tell the DHCP server, I’m moving to a different subnet. So please release that IP address if somebody else comes along and needs an IP address, and you’ve run out. You can absolutely give them mine to use.

There are two important timers to consider when we’re discussing the DHCP renewal process. The first timer is the T1 timer. This T1 timer is how often your workstation is going to check in with the DHCP server, and confirm that the IP address is still being used on the network and effectively restart the timer on the lease. It does this when the lease time gets down to 50%.

So if you’ve been allocated an IP address with a lease time of eight days, your workstation will wait four days and then check in with the DHCP server. At which time the timer is set back to eight days again. But what if the DHCP server isn’t there anymore? What if it goes down? What if there’s a problem communicating to it? This is where we use the T2 timer.

If that first server is down, we’re going to try communicating with any DHCP server on the network. But we’re going to wait past the 50% mark, past 60, past 70, all the way up to 87.5% of the lease time. That’s 7/8 of the lease time. So we’re waiting almost until the end in the hopes that that original DHCP server will somehow come back on the network or be available again.

Visually we can see it this way. Let’s say that we’ve been given a lease time of eight days. That means if we calculate this, the T1 timer is going to be four days. That’s an easy calculation– 50% of the time. And in this case, it’s an easy calculation again. The T2 timer is going to be 7/8 of the lease time, or seven days.

So if we’re going through the normal operation, when we hit 50% we’re going to be in this renewable period where we need to check back in. And if the DHCP server is available, we can check back in, renew the lease timer, and our T1 is reset back to 0. But let’s say at this point the original DHCP server goes away for some reason. We’re going to go past our 50% mark, and we’re going into and past the renewal period.

Now we’re in that last eighth of the time. And we’re in what we call a rebinding period where now we just need an IP address from anybody. And at that point we’re going to check in with any DHCP server, renew that, and get an updated T1 timer. Everything resets and we can start the lease time over again. All of the devices on your network are going to be using this lease timer to check in with your DHCP server, so that they can always have an IP address configured in their devices.

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Category: CompTIA Network+ N10-006

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