Weird problem - network printers

Ok, two pretty identical setups: one at home, one at work. Multiple pc's, cable moden, 4 port router + wifi, network printer.

The printer is plugged into the router and given an address (like 168.192.1.4). The weird thing is that the address will change to something like 168.192.1.7 without me doing anything. Windows doesn't correct this mistake (xp at work, vista at home).

My limited network knowledge thought that the wired ports were constant. ie.
port 1 = 168.192.1.1
port 2 = 168.192.1.2
port 3 = 168.192.1.3
port 4 = 168.192.1.4
but obviously that isn't the case. The hardware is completely different at the two locations. (belkin and netgear routers, oki and samsung laser printers).

Any thoughts on what is going on? I'm pretty sure that if I look hard enough into the router's setup I can lock a device's port, but i'm more interested in learning why this is happening. It happens about once a month or less.

wow, thanks for the fast reply.

I'm reading up more on DHCP now. I'm not sure what value to assign the printer but I'm pretty confident i can figure it out.

Still not sure why it would get assigned .7 when I only have 5 total devices attached but I guess it doesn't matter much.

DHCP assignments generally are good for a while, like a few days. removing devices doesnt remove the assignment until the time on the assignment runs out. As long as you set the static IP address of the printer outside of the dhcp range you will be fine.