FreakShow,
When you connect to the Internet, you are NOT connecting directly to the Internet. Instead, you connect to an Interent Service Provider (ISP), who is directly connected to the Internet.
Every computer on a TCP/IP network has an IP address. (Just about every network including the Internet is a TCP/IP network). There is a limited number of IP addresses, and no two computers on a network can have the same ones.
An ISP has routers that interface directly to the Internet. Routers tell Internet traffic where to go, based on the IP address of the traffic.
Your computer is therefore considered to be "behind the router", because the routers on the Internet only see other routers, not your PC.
Your ISP may use a DHCP server to assign IP addresses automatically. DHCP is "dynamic host configuration protocol" - basically means "gives out IP address
automatically".
The IP addresses assigned by the DHCP are the IPs of the PCs, and may change. But it does not matter - alll I will see from here is teh address of your ISp's router, which does not change. (It is not allowed to change because other routers have to know where it is).
NAT is something caled Network Address Translation. Some ISPs run it because it allows them to use fewer IP addresses.....the ones behind the router do not have to be unique - only the ones on the router itself.....
Better?