DHCP Reservations – CompTIA Network+ N10-006 – 1.3

| March 22, 2015


Although many DHCP addresses can be assigned from a large pool, you can also reserve IP addresses for certain devices. In this video, you’ll learn about DHCP reservations and how they are configured in a DHCP server.

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When you’re on a network that’s providing your IP configuration through DHCP, it may be deciding what IP address you get using a number of different mechanisms. The most common one is dynamic allocation. This is the method that’s used when you first walk into a network for the first time and you plug in your device, and you try to connect to the network.

Somewhere on that network there will be a DHCP server. The network administrator has configured this DHCP server with a very large pool of IP addresses. And it simply provides you with one of those from the pool. After you pack up your system and you leave, maybe you leave for a week, and now the lease timer on that IP address has ended.

The DHCP server will take that IP address back in market as being available. But the DHCP server doesn’t give away that IP address, at least not initially, it waits to see if you come back. And one of the things you’ll notice is that if you do leave a network and come back and plug back in, you’ll probably get the same IP address again. This is something called automatic allocation.

Your DHCP server is going to keep a list of all the Mac addresses that were connecting to the network, and the IP address that was assigned to that Mac address. And if it sees that Mac address appear again, you’re going to get exactly the same IP address. It’s so much easier to manage things if you know that you’re going to have the same IP address every time you connect to the network.

Another type of IP allocation is called a static allocation. What if you didn’t want the IP address to be something that was randomly chosen from a pool? What if you wanted a device to be guaranteed to have exactly that IP address every time it connected to a network? And that method is called static allocation, because you are automatically and statically configuring an IP address and associating it with a very specific device.

Your DHCP server will have a table of Mac addresses, and you’ll configure what IP address should be associated with that Mac address. And if that Mac address is seen by the DHCP server, it will automatically assign that. That IP addresses is also not going to be used in the pool, so that you can be assured that it will be held back and statically assigned every time that IP address hits the network. You may see this also referred to as static DHCP assignment, static DHCP address reservation, or IP reservation.

You can perform this static reservation on a number of DHCP devices, even some that are integrated into all-in-one devices. This is one from a device that I have that’s a simple business router. And you can see that the address reservation at the bottom shows the Mac addresses for my devices.

The Prometheus device has this Mac address, and the Odyssey device has this Mac address. And I’ve told my DHCP server when these Mac addresses hit the network, these are the IP addresses that I would like you to assign to these every single time. This way you can configure the DHCP server to take care of not only the dynamically assigned people on your network, but also servers and other devices that should always have a static address.

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Category: CompTIA Network+ N10-006

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