DHCP Options – CompTIA Network+ N10-006 – 1.3

A DHCP assignment can configure more than just an IP address, subnet mask, and default gateway. In this video, you’ll learn about DHCP options and how they are used on our modern networks.

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We commonly think of DHCP as automatically providing our devices with an IP address, and a subnet mask, and a default gateway. But there are so many different kinds of devices out there, and they may need a very different set of IP configuration settings. These IP configuration settings have been taken into account as part of the DHCP RFC. We call these additional settings the DHCP options.

If you were looking at the older BOOTP, you may see them referred to as vendor extensions. It’s almost a better description of what they are, because they extend the capabilities of DHCP. And they’re based on what a particular vendor might need in their particular device.

There are so many options available. You’ve got 254 usable options between 0 and 255. We don’t use 0. We don’t use 255. That gives us 254 options in the middle that we can configure.

We’re, of course, going to configure the very common options on our DHCP server of our IP address and subnet mask and default gateway. But we’re also going to configure those additional options on our DHCP server as well. You do have to check with your DHCP server because not all DHCP servers support option configuration. So you have to check with the documentation of your DHCP server and make sure that you can configure those.

One good example of using these DHCP options is with Voice over IP devices. You might have a Voice over IP phone on your desk. And just like your computer, your Voice over IP phone does not have an IP address configured in it. It has to talk to a DHCP server and get a DHCP address for that phone. But the phone also needs to check in with the call server. And of course, the phone doesn’t know where the call server is either.

So in your DHCP server you would set an option 129, which is the call server’s IP address. And that way when the phone gets an IP address, it also gets an option 129 that has the IP address of the call server. So that it can then check in with the call server and become an active phone on your network.

Here’s the DHCP options that are on my Windows Server 2008. This is my DHCP server. And you can see not only can I set the scope and the IP address settings for that particular subnet, I simply choose the server options. And then I can add any server options I would like inside of that server options field.

If a machine receives an option that it normally wouldn’t use, it simply ignores it. But it also ensures that things like my Voice over IP phones are able to receive all of the information they need to be able to work properly on my network.