What is DHCP?

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Summary: DHCP is the protocol computers use to request and receive a "dynamic" IP address.

DHCP stands for "Dynamic Host Configuration Protocol".

So I can hear you thinking, "great ... what's that?"

In a nutshell, it's the request your computer makes and the response it receives that assigns it a "dynamic" IP address.

Let's look at that a little more closely.

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First, a refresher. Every computer on the internet has an "address". That address is just a number, but it uniquely identifies that computer; no other computer on the internet can have the same address.

There are two types of IP addresses: static and dynamic. Static IP addresses are just that - unchanging. They refer to a particular computer, whether that computer is turned on or not, connected or not. Most typically domain names like "ask-leo.com" map to a single static IP address.

Dynamic IP addresses are assigned to a computer on-the-fly. The most common scenario is dial-up. When you dial-up to your ISP, part of the connection sequence is your computer asking for an IP address, and your ISP assigning it one of the addresses it has available. When you disconnect, that IP address is returned to the list of available addresses and may be reused by another computer when it dials in.

Because most home users tend to turn their computers off, even broadband connections such as DSL and Cable continue to use dynamic addresses. If your computer is off, you don't need an address so someone else might get to use it.

Dynamic addresses are also used by NAT routers. Not necessarily on their internet connection, though it can, but rather on the local area network side. Each computer that you connect to your router will ask for an IP address, and that router will respond with an IP address that's available within your LAN.

The actual DHCP protocol is fairly simple. The computer in need of an IP address broadcasts a request, meaning it sends out a request to anyone who'll listen. That, in essence, says "Hey, anyone? I need an IP address!" By definition on any network (or more correctly, sub-network), there should at most one device who's job it is to answer back "Sure, here ya go, have this one." That's a DHCP server. Along with that answer is additional information as well, such as what machines to ask for domain name (DNS) look-ups and what address to forward all outgoing network traffic, too (the gateway).

In Windows, if your computer doesn't get a response from a DHCP server within a certain amount of time, it will give up or it may fall back to only asking every few minutes. It may "make up" an answer as well. In fact if your IP address begins with "169.", that's probably exactly what's happened; no IP address was assigned, and Windows made one up. And I'll bet your networking and internet don't work.

Related:

• Ask Leo! - What's the difference between a Hub, a Switch and a Router?

• Ask Leo! - How should I set up my home network?

• Ask Leo! - All networking articles.

• Amazon.com - Best Selling books about PC Networking.

• Amazon.com - Best Selling books about DHCP (typically very technical).

• The DHCP FAQ with much more technical detail.

Article 330 | Posted June 17, 2004