They all have the same external IP because the router you are using does NAT, or IP Masquerading. Essentially, you have an internal LAN connected to one of the router's NICs and there's an external one which connects to the Internet. Everything from outside your LAN sees the external NIC and it's your router which figures out where to send packets inside the LAN - so, unless you port-forward requests on certain ports of the router to specific machines on the LAN they are dropped. It's a nice security feature as well: only computers on the LAN can instantiate connections (if there's no user defined port forwarding going on). NAT is also a nice way around the shortage of IPv4 addresses.

Well, that's the gist anyway - check wikipedia or google at large for more info, I suppose.