Chapter 12 NAT
USG20(W)-VPN Series User’s Guide
262
Figure 171 LAN to LAN Traffic
The LAN SMTP server replies to the USG’s LAN IP address and the 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 172 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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