Chapter 21 Network Address Translation (NAT)
ZyWALL 5/35/70 Series User’s Guide
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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.
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.
21.5.1 Default Server IP Address
In addition to the servers for specified services, NAT supports a default server IP address. A
default server receives packets from ports that are not specified in this screen.
" If you do not assign a Default Server IP address, the ZyWALL discards all
packets received for ports that are not specified here or in the remote
management setup.
21.5.2 Port Forwarding: Services and Port Numbers
The ZyWALL provides the additional safety of the DMZ ports for connecting your publicly
accessible servers. This makes the LAN more secure by physically separating it from your
public servers.
Use the Port Forwarding screen to forward incoming service requests to the server(s) on your
local network.
The most often used port numbers are shown in Appendix F on page 813. Please refer to RFC
1700 for further information about port numbers.
21.5.3 Configuring Servers Behind Port Forwarding (Example)
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.
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