Some routers do that.
I have a Brother HL 5250DN printer connected to my linksys router. Whenever I attempt to print from my Toshiba laptop(L355D-S7815), I find that I have to go in and change the port from 192.168.1.100 to 192.168.1.101 or vice versa. It seems whatever I use for a port needs to be changed the next time I use the laptop. My other Toshiba laptop(P205D-S7802) does not seem to have this issue. The desktop works fine with the printer. What is happening? Thanks.