Gateway none - Manual (Page 60)

4-24
CHAPTER 4
The WAN interface must have a fixed IP address to utilize
this function. For example, if you set Type/Public Port to
TCP/80 (HTTP or Web) and the Private IP/Port to
192.168.2.2/80, then all.NETWORKING AND CLIENT
SERVICES 4-18 HTTP request from outside users will be
transferred to 192.168.2.2. Therefore, by just entering the IP
Address provided by the ISP, Internet users can access the
service they need at the local address to which you redirect
them.
Some of the more common TCP service ports include:
HTTP: 80, FTP: 21, Telnet: 23 and POP3: 110.