Chapter 13 NAT
ZyWALL USG Series User’s Guide
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The LAN user’s computer then sends traffic to IP address 1.1.1.1. NAT loopback uses the IP address of the
Zyxel Device’s LAN interface (192.168.1.1) as the source address of the traffic going from the LAN users to
the LAN SMTP server.
Figure 308 LAN to LAN Traffic
The LAN SMTP server replies to the Zyxel Device’s LAN IP address and the Zyxel Device changes the
source address to 1.1.1.1 before sending it to the LAN user. The return traffic’s source matches the
external destination address (1.1.1.1). If the SMTP server replied directly to the LAN user without the traffic
going through NAT, the source would not match the external destination address which would cause
the LAN user’s computer to shut down the session.
Figure 309 LAN to LAN Return Traffic
192.168.1.21
LAN
192.168.1.89
Source 192.168.1.89
SMTP
NAT
Source 192.168.1.1
SMTP
192.168.1.21
LAN
192.168.1.89
Source 1.1.1.1
SMTP
NAT
Source 192.168.1.21
SMTP
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