Interview question of the week: what is DHCP, and what is it used for?

There are some questions that are guaranteed to be asked during a help desk interview. This is one of them.

Image by Gerd Altmann from Pixabay

A very basic understanding of DHCP is necessary for being a successful help desker. You don’t need to know how to deploy a DHCP server, but you should at the very least know the following basics before you go to your first interview.

I’ll give you a sample answer at the end of this post, but a little bit of theory is helpful first.

DHCP stands for “Dynamic Host Configuration Protocol.” Each computer connected to a network needs to have a way to be found by other computers. On TCP/IP (which is the protocol that the Internet runs on) networks, computers are identified by their IP addresses. IP stands for “Internet Protocol.”

So how does your computer get an IP address?

Well, you could manually set it, but this is tedious even on a small network where you’ll only have fewer than ten or so computers. For example, imagine that every time you came home your phone couldn’t automatically connect to your WIFI. Instead, you had to manually input an IP address every time. Bummer! And while this would be annoying on a small network, on larger networks (like a large corporation or even on the Internet) this would be completely unworkable.

Here’s where DHCP comes in. DHCP is what dynamically assigns you an IP address. The DHCP server knows which IP addresses are available and assigns one to you with nothing needed on your end.

You’ve interacted with DHCP thousands of times without knowing it. Each time you connect to a network automatically you are automatically being assigned an IP address by DHCP. If DHCP didn’t exist, you’d have to ask an administrator for an IP address, a subnet mask, the gateway’s address, and which DNS server to use. If you got any of those wrong, you wouldn’t have connectivity.

This is important for IT support because you’ll eventually have to troubleshoot a connectivity issue that will be related to DHCP.

For example, a user may not be able to connect to the network. For the purposes of this example, let’s say you’ve troubleshot a bit already and determined that the physical hardware is working and everything on the software side of things is set correctly. The problem is clearly coming from outside of the user’s laptop, but everyone else seems to be connected to the network just fine.

If the DHCP service on the DHCP server has stopped functioning correctly, this could be the cause. What normally happens is that a computer will first connect to the network and request an IP address. When the DHCP server is working properly, all this will happen transparently and everything will just work. However, if there’s a problem with DHCP, the computer will reach out and get no answer. In these cases, the computer will give itself an IP address that starts with 169.254. This is called an “APIPA” IP address.

Do this right now: press the start button and type “CMD”. At the command prompt, type “ipconfig”. You should see something like this.

See where it says IPv4 Address? If the IP started with 169.254, that would be your clue that DHCP wasn’t running. In that case, you would ask your server administrator to restart the DHCP service. As you may know already, restarts fix 95% of problems in IT!

If DHCP is working properly, you’ll see an IP address that most often starts with 192.168. This range of IP addresses is reserved for private use and can’t be used on the Internet. (There are other private ranges, but this is the most commonly used one.) Your home router might give you an IP address of 192.168.1.5, for example, and my home router might give me the same one. However, for our two computers to communicate with each other, we’d be using our router’s externally facing IP addresses, which are unique. (And yes, our routers’ IP addresses are dynamically assigned using DHCP that is managed by your Internet Service Provider.)

When you’re asked about DHCP during your interview, you should at least be able to talk about what DHCP stands for (Dynamic Host Configuration Protocol), the difference between a static IP address and a dynamic IP address, what private IP addresses are, and what an APIPA IP address indicates.

A sample answer might be:

“DHCP stands for Dynamic Host Configuration Protocol. This is what dynamically assigns IP addresses to devices so they can connect to a network. When DHCP is working fine, an IP address is automatically assigned and a device is assigned a private IP address. If it’s not working well, a device will assign itself an APIPA IP address which starts with 169.254.”

This answer is short and sweet, but it is more complete than any interviewee has ever been able to give me when I’ve asked it. Knowing how to answer this question is a very easy way for you to stand out from your competitors and show off some of your knowledge.

With each post, I cover a new topic to help you get your start (or keep progressing) in your IT career. If it’s your first time visiting this blog, start here. Or, see all my posts about interview questions you should be able to answer.

Author: Silicon Wanderer

I'm a merry wanderer on the path to financial independence through IT. I'm doing it, and I want to show you how you can to!

2 thoughts on “Interview question of the week: what is DHCP, and what is it used for?”

  1. During my first technical interview ever I was asked about DHCP. I like to use the acronym ‘DORA’ to remember how DHCP works: Discover, Offer, Request, Accept. This describes the relationship between the client and the DHCP server.

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s