1. Your external address is the address of your Internet conection, your firends must use this address to connect to your server. Find out what is your external ip in;
http://whatsmyip.org/.
2. Your router's address is 192.168.1.1 . You use this address only to define gateway or DNS server in the setup of the network-connection on your PC.
3. Your internal IP will be of type 192.168.1.(2..254). This address can be assigned by the router with DHCP when this is so defined in your network-conection. But you should setup a fixed address in your PC, like 192.168.1.2
Then you have to forward in the router the port you use (81) to your in internal IP (192.168.1.2), so that the router knows to which internal ip (or computer on the lan) he has to forward requests on port 81.
Look at portforward.com for more.
