If you have a Linux host connected to the Internet and.. [Learn about Cybersecurity]
if you have internal computers running TCP/IP connected that are connected to this Linux box via on a network, and/or .:: www.rctech.net ::. .:: mindef.gov.bn ::. [What people say about SMTP]
if your Linux host has more than one modem and acts as a PPP or SLIP server connected to .:: www.just.edu.jo ::. other computers, and these machines do not have official or public assigned IP addresses (i.e. addressed with private TCP/IP numbers). .:: podcasts.apple.com ::.
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. .:: paper.wf ::.