ZyXEL Communications ZyWall ATP series User Manual page 339

Chapter 12 NAT
Figure 230 LAN to LAN Traffic
NAT
Source 192.168.1.1 Source 192.168.1.89
SMTP
SMTP
LAN
192.168.1.21 192.168.1.89
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
original 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 original destination address which would cause the
LAN user's computer to shut down the session.
Figure 231 LAN to LAN Return Traffic
NAT
Source 192.168.1.21
Source 1.1.1.1
SMTP
SMTP
LAN
192.168.1.21 192.168.1.89
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