[MovedGoalPosts]

Something odd with your laptop's settings if it isn't getting an IP address from the router when you havea wired connection to it.

have a look at your network card settings and make sure that eveything, especially the TCP/IP settings for it are set to auto. Don't know if there is an equivalent for the mac, but on a PC trying a command prompt iprelease and ipconfig can force a reset, where something has got stuck.

Make sure you power off the laptop when you swap it's connection between modem and router. Router should be running before you try and connect the laptop.