ZyXEL Communications P-660R-11V3 User Manual

Chapter 10 Network Address Translation (NAT)

P-660R-T1 v3 User’s Guide

LAN computers usually have DHCP-assigned private IP address that cannot be accessed 
directly from the WAN. Use this screen to allow the ZyXEL Device to forward traffic to the 
servers on the LAN.
You may enter a single port number or a range of port numbers to be forwarded, and the local 
IP address of the desired server. The port number identifies a service; for example, web 
service is on port 80 and FTP on port 21. In some cases, such as for unknown services or 
where one server can support more than one service (for example both FTP and web service), 
it might be better to specify a range of port numbers. You can allocate a server IP address that 
corresponds to a port or a range of ports.
The most often used port numbers and services are shown in 

. Please 

refer to RFC 1700 for further information about port numbers. 

"

Many residential broadband ISP accounts do not allow you to run any server 
processes (such as a Web or FTP server) from your location. Your ISP may 
periodically check for servers and may suspend your account if it discovers 
any active services at your location. If you are unsure, refer to your ISP.

Let's say you want to assign ports 21-25 to one FTP, Telnet and SMTP server (A in the 
example), port 80 to another (B in the example) and assign a default server IP address of 
192.168.1.35 to a third (C in the example). You assign the LAN IP addresses and the ISP 
assigns the WAN IP address. The NAT network appears as a single host on the Internet.

Figure 36   Multiple Servers Behind NAT Example

The following table summarizes the settings described in the above example.

Table 23   Multiple Servers Behind NAT Example

A

FTP

21-22

192.168.1.33

B

Telnet

23

192.168.1.34

C

SMTP

25

192.168.1.35

A=192.168.1.33

C=192.168.1.35

B=192.168.1.34