ZyXEL Communications USG40 User Manual page 394

Chapter 13 NAT
Figure 272 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 ZyWALL/USG's LAN IP address and the ZyWALL/USG 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 273 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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