Network Address Translation; Overview - ZyXEL Communications NBG-418N User Manual

10.1 Overview

This chapter discusses how to configure NAT on the NBG-418N.
NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a host in
a packet. For example, the source address of an outgoing packet, used within one network is
changed to a different IP address known within another network.
Each packet has two addresses – a source address and a destination address. For outgoing packets,
NAT maps private (local) IP addresses to globally unique ones required for communication with
hosts on other networks. It replaces the original IP source address in each packet and then
forwards it to the Internet. The NBG-418N keeps track of the original addresses and port numbers
so incoming reply packets can have their original values restored. The following figure illustrates
this.
Figure 72 NAT Example
A: 192.168.1.33
FTP, Telnet, SNMP
Ports 21 to 25
B: 192.168.1.34
Port 80
C: 192.168.1.35
For more information on IP address translation, refer to RFC 1631, The IP Network Address
Translator (NAT).
Note: You must create a firewall rule in addition to setting up NAT, to allow traffic from
the WAN to be forwarded through the NBG-418N.
NBG-418N User's Guide
C
HAPTER

Network Address Translation

LAN
192.168.1.1
WAN
IP address
assigned by ISP
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