SMC Networks ADSL Barricade SMC7204BRA - Manual (Page 161)

Modifying Port Settings
163
Modifying Port Settings
Overview of IP port numbers
The header information in an IP data packet specifies a
destination port number. Routers use the port number along
with the specified IP addresses to forward the packet to its
intended recipient.
For example, all IP data packets that the ADSL Barricade
receives from the Internet specify the same IP address (your
public IP address) as the destination. However, depending on the
port number contained in the data packets, the ADSL Barricade
may pass the packet on to its embedded Web or Telnet servers,
or to another computer on the network.
The Internet community has developed a list of common server
types such as HTTP, Telnet, e-mail, and many others, and has
assigned a unique port number to each. These are not mandatory,
but are useful in promoting communication between separately
administered LANs.