Port Mapping (Forwarding, Virtual Server)
This feature goes by many names, but they all allow you to open holes (ports) in your firewall. You'll need to do this for most any Internet applications that depend on the ability of someone on the WAN (Internet) side of your router to send a data request to a computer on your LAN.
There are a few ways that manufacturers implement port mapping, and what you need will depend on what sort of applications you use. Let's take a look at the different types of port mapping features.
Tip: "Port" in this case, refers to a logical communication connection, not a physical socket. You can think of ports as channels, with certain protocols using particular channels. An example is that the HTTP protocol used for WWW traffic uses Port 80.
Static Single Ports
Figure 4 shows an example of the simplest form of port mapping. You must map each port used by an application to the IP address of the computer that the application is running on. Some routers allow you to specify either the protocol used for the mapping (TCP or UDP). Others automatically map the port for both protocols.
Figure 4: Port Forwarding Example
NOTE: You can statically map a specific port to only one IP address. This means that if you have multiple users who want to use the same application, or multiple servers of the same type, each copy of the application or server would need to use a different port. Some applications allow this to be done, and others don't.
If you have only a few applications and they use only one or two ports each (i.e. running a web or FTP server) this method should be fine. Although the number of single port maps vary from manufacturer to manufacturer, you'll typically get somewhere around ten mappings.
Static Port Ranges
Similar to single port mapping, this option lets you map a range of ports in each mapping. Each mapping still applies to only one IP address, however. This option gives you the ability to handle applications that use a lot of ports such as games and audio/video conferencing. Again, the number of mappings varies from product to product, with ten or so being typically offered.