Certainly you don’t need to know anything about port numbers if you never plan to allow external users to access your network, or if you don’t plan to control the types of Internet services that your internal users can use. However, if you do plan to make use of either feature, you’ll need to know something about port numbers.
Different types of applications use different port numbers to communicate. Port numbers come in two flavors, namely TCP and UDP. Transmission Control Protocol (TCP) is a reliable protocol used by some applications (such as Web, FTP, and Email servers), while User Datagram Protocol (UDP) is a faster (but unreliable) protocol used by services like DNS. You don’t get to choose which is used – the specifications for different services define which protocol is used which individual applications.
A total of 65536 TCP/UDP port numbers exist. Certainly no one could remember all of them, but some of them are much more common than others. For example, the list below outlines some of the port numbers used by common services:
HTTP (Web servers) – TCP 80
FTP (FTP servers) – TCP 21
SMTP (Email servers) – TCP 25
POP3 (Email servers) – TCP 110
DNS (Name resolution) – UDP 53
This is far from a comprehensive list, but gives you the idea. So, if you plan on having your own internal FTP server that should be accessible from the Internet, port forwarding would need to be enabled on your router for TCP ports 20 and 21. If you’re using ICF, a definition for FTP already exists which you can simply check off to accomplish the same task. For a complete and very comprehensive list of port numbers, see http://www.iana.org/assignments/port-numbers.