In TCP/IP and UDP networks a port is a 16-bit number used to identify which
application program (usually a server) incoming connections should be delivered to.
Some ports have numbers that are pre-assigned to them by the IANA (the Internet
Assigned Numbers Authority), and these are referred to as “well-known ports”. Servers
follow the well-known port assignments so clients can locate them.
If you wish to run a server on your network that can be accessed from the WAN (i.e.
from other machines on the Internet that are outside your local network), or any
application that can accept incoming connections (e.g. Peer-to-peer/P2P software such
as instant messaging applications and P2P file-sharing applications) and are using NAT
(Network Address Translation), then you will usually need to configure your router to
forward these incoming connection attempts using specific ports to the PC on your
network running the application. You will also need to use port forwarding if you want
to host an online game server.
The reason for this is that when using NAT, your publicly accessible IP address will be
used by and point to your router, which then needs to deliver all traffic to the private
IP addresses used by your PCs. Please see the WAN configuration section of this
manual for more information on NAT. The device can be configured as a virtual server
so that remote users accessing services such as Web or FTP services via the public
(WAN) IP address can be automatically redirected to local servers in the LAN network.
Depending on the requested service (TCP/UDP port number), the device redirects the
external service request to the appropriate server within the LAN network.
7.6.1 Port Forwarding
Because NAT can act as a “natural” Internet firewall, your router protects your
network from being accessed by outside users when using NAT, as all incoming
connection attempts will point to your router unless you specifically create Virtual
Server entries to forward those ports to a PC on your network.
When your router needs to allow outside users to access internal servers, e.g. a web
server, FTP server, Email server or game server, the router can act as a “virtual
server”. You can set up a local server with a specific port number for the service to
use, e.g. web/HTTP (port 80), FTP (port 21), Telnet (port 23), SMTP (port 25), or
POP3 (port 110), When an incoming access request to the router for a specified port is
received, it will be forwarded to the corresponding internal server.