Some of you will have had occasion to poke around in the network settings of your Mac, iPhone or iPad and you may have come across the term DHCP and wondered what it meant. In most situations, you will find the network configuration of your device will be set to Configure Using DHCP. So what is DHCP and what does it do?
DHCP stands for Dynamic Host Configuration Protocol. Sounds complicated right? It's not! What it means is simply this: instead of you needing to know all the settings that your device will need to connect to the network and internet, there is some device already on the network that can give you those settings automatically so that you just don't need to know!
Usually, the device that does this is the router that connects you to the internet. Your own device, say your Mac or iPhone, simply sends out a message on the network asking if any device out there can provide it with the settings it needs. Usually there is one and it sends back a message which says "Hi! I'm your router and I'd be pleased to help. Here are the settings you need. Have a nice day!"
Once this happens, your device is automatically configured with those settings and Voila! you are connected to the internet. Clever eh?
Sometimes, if there is no DHCP Server available (the device that gives out this information, such as a router), your device will not be able to get these settings and will not be able to get on the internet. On the Mac, you will usually see this indicated as an exclamation mark in the Airport menu (on the menu bar). If you look further by going to Apple Menu -> System Preferences and then choosing the Network pane, you will see more indicators that this is the problem.
As you can see in the image above, the status will show that it has a self-assigned IP address and the IP address will usually start with 169.xxx.xxx.xxx in this case. When you see this, you can be sure that the problem is that your device is not able to get an IP address from the router (or DHCP server). There are many reasons why this could be the case, but it's an excellent first step in finding the problem!