Virtual Servers

Just another name?

As the name implies, virtual servers are not physical (real) servers. The term is used to refer to the assignment of a port to a server so that the router knows how to redirect network signals. Accordingly, setting up virtual servers is close to saying forwarding ports.

Assigned ports

There is nothing to stop a host from assigning any port to any server. However, you should do it with care. The computing community, through IANA (Internet Assigned Numbers Authority) has adopted a standard to take the headache away from application authors. As a result many ports numbered below 1024 (stateful or low ports) have been assigned. Examples of some well known ports and the corresponding server (assigned by IANA) are:

Port	Protocol	Server
80	TCP	HTTP
21	TCP	FTP
25	TCP	SMTP
110	TCP	POP
143	TCP	IMAP
3306	TCP	MYSQL
389	TCP	LDAP

Unassigned ports

Most ports in the "high" (stateless) range are not assigned. This is where users can do their own assignments. Of course, we know these:

Port	Protocol	Server
6900	TCP	login
6121	TCP	char
5121	TCP	map

How to set up virtual servers?

As in port forwarding, the dependence on hardware makes difficult for someone to tell you how to configure your router. Even the same model may differ. An understanding of routing will help. The basic line (rule) is:

allow source [public IP] [port] destination [private IP] [port] protocol

A real world example

This guide is based on DLink DI-704 router (with print server). Although proven, you should not assume that this guide will work even if you have the same model.