P-873HNUP-51B Support Notes
All contents copyright (c) 2011 ZyXEL Communications Corporation.
NAT Port Forwarding
NAT Introduction
What is NAT?
The NAT (Network Address Translation-NAT RFC 1631) is the translation of an
Internet Protocol address used within one network to a different IP address known
within another network. One network is designated as the
inside
network and the
other is the
outside
. Typically, one company maps its local inside network addresses
to one or more global outside IP addresses and "unmaps" the global IP addresses on
the incoming packets back into local IP addresses. The IP addresses for NAT can be
either fixed or dynamically assigned by the ISP. In addition, you can designate servers,
e.g., a Web server and a Telnet server, on your local network and make them
accessible to the outside world. If you do not define any servers, the NAT offers the
additional benefit of firewall protection. In such case, all incoming connections to
your network will be filtered out by the CPE, thus preventing intruders from probing
your network.
For more information on the IP address translation, please refer to RFC 1631,
The IP
Network Address Translator (NAT)
.
Port numbers for some services:
Service
Port Number
FTP
21
Telnet
23
SMTP
25
DNS (Domain Name Server)
53
www-http (Web)
80
