ZyXEL Communications NXC2500 User Manual page 145

Chapter 11 NAT
The LAN user's computer then sends traffic to IP address 1.1.1.1. NAT loopback uses the IP address
of the NXC'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 72 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 NXC's LAN IP address and the NXC 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 73 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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NXC2500 User's Guide