What makes the IP change for a cable modem?

[redss]

I have comcast cable internet, and I have had the same external IP address for months, with a 4-port hardware router behind my cable modem.

I discovered that my IP address changed when I took the router out and plugged the computer direct to the cable modem. And it changed again when I put my voip gateway between the cable modem and computer. THEN I put the router back, and my IP address changed back to the same one it had been when the router was there before!

Since my cable modem is the first thing seen from the internet and it hadn't changed (except for having to power off/on), Why would something AFTER the cable modem (like a router) change what IP address the cable modem is assigned?

 

[hispeedtech]

Two things occur to change the ip address in this situation.
First, Comcast cable is a DHCP connection. It "authenticates" the user through a machine's physical address instead of using a username and password like a PPPoE connection (dsl is usually this type)
Second, the modem you have is a "dumb" modem. It doesn't send Comcast a physical address but sends the routers physical address. Your router then assigns a subnetwork address to your pc. When you plugged your pc directly into the modem, it used the pc's physical address and recieved a new ip address from comcast. When you plugged in your VOIP gateway it pulled the physical address from it and recieved a new ip address, etc., etc.
Comcast also changes your ip address from time to time. I've seen as often as 10 days, but I've had mine changed only 4 times in almost 2 Years! You DON'T have a static ip address (unless your a very old customer, then you might have).
I wouldn't change the setup around too often, the network checks for changed physical addresses at a given location and will eventually refuse to give you a valid ip. You then will have the possibility of a physical address mismatch (comcast is looking for the address for your VOIP when your router's hooked up). Not too bad if they have your pc or router address, a royal pain in the butt if they have your VOIP address!
Hope this helped