If you have a Linux host connected to the Internet and..
if you have internal computers running TCP/IP connected that are connected to
this Linux box via on a network, and/or
if your Linux host has more than one modem and acts as a PPP or SLIP server
connected to other computers, and these machines do not
have official or public assigned IP addresses (i.e. addressed with private
TCP/IP numbers).
If you want those OTHER machines to communicate to
the Internet without spending extra money to acquire additional Public /
Official TCP/IP addresses from your ISP, then you should either configure
Linux to be a router or purchase an external router.