If you are setting up a home network and find that your computer works, but when you plug in a wireless router it will not get online, THIS is what is happening.  Understanding it gives you the steps to work around the problem.

We are assuming Comcast here, they seem to have the most finicky modems anyways.

DHCP is the protocol that is at work when you ‘get an ip’ from Comcast, or any other server.  when you plug a laptop into a wireless router, you ‘get an ip’ – the laptop is speaking the protocol of DHCP (dynamic host configuration protocol) and asking ‘hey – anyone out there want to give me a configuration?’ then the router answers, ‘sure, have this one’

It is based on the MAC address (hardware physical ID) of the network adapter, every network adapter on the planet has a UNIQUE MAC address.

Your computer detects ‘link’ (plugged in, has voltage) and windows realizes it needs an IP and starts asking out that adapter. 

The first DHCP server to reply (there become problems with more than one unless a very special circumstance) will hand out the address

In a home network, the wireless router hands your computer an address, and Comcast/ISP hands your ROUTER an address.  both of these have to work before you can get online. 

What we see when your computer can plug in directly, but the wireless router won't get you online:

1.       Laptop network adapter has mac 000000001 for example, it connects to Comcast, says: ‘hi, I’m 0000000001, can I have an IP?’

2.       Comcast says: ‘hi 00000000001, sure, you are: 171.10.90.55, and you can have that for 5 days, then check back’

3.       Computer is online

4.       You change to the wireless router, it says: ‘hi, I’m 010101011, can I have an ip?’

5.       Comcast thinks: they must be plugging in a SECOND device, they can’t have 2 ip’s – I’m not answering.

6.       We reboot devices, play around, etc – Comcast still thinks 2 devices are plugged in and only wants to answer to 000000001

7.       You plug in the network cable to the wireless router, and click ‘clone mac’ – this makes the ROUTER pretend to have the mac 00000001

8.       You plug the network cable back in to the Comcast modem and the ROUTER says: ‘hi, I’m 00000001, can I have an ip?’

9.       Comcast says: ‘hi again, you have one for 4.99 more days, it’s 171.10.90.55’

10.   You are online. 

11.   This all had to happen because the Comcast modem wasn’t smart enough to figure out you had unplugged the machine with MAC 00000001

Fun huh…

Here is a slideshow I found on DHCP - old and backs me up mostly

http://www.cisl.ucar.edu/nets/presentations/DHCP-in-SCD/sld001.htm