Due to the explosion in the expansion of the Internet over the last few years, free IP addresses are in short supply. They are only assigned very sparingly. NAT is used where company networks are to be linked to the Internet. The company network is linked to the Internet via a NAT-compatible Router. This operates internally with its own IP address space that is independent of the Internet. The network can only be addressed from the outside via a single (or just a few) IP address(es). The port number on a received TCP/IP packet is used to pass the packet on to a specific internal network subscriber.