ZyXEL Communications UAG5100 User Manual page 191

Chapter 15 NAT
Figure 117 LAN to LAN Traffic
NAT
Source 172.16.0.1 Source 172.16.0.89
SMTP
SMTP
LAN
172.16.0.89
172.16.0.21
The LAN SMTP server replies to the UAG's LAN IP address and the UAG 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 118 LAN to LAN Return Traffic
NAT
Source 172.16.0.21
Source 1.1.1.1
SMTP
SMTP
LAN
172.16.0.89
172.16.0.21
UAG5100 User's Guide
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