ZyXEL Communications 310 User Manual page 226

Chapter 13 NAT
Figure 129 LAN Computer Queries a Public DNS Server
DNS
xxx.LAN-SMTP.com = 1.1.1.1
xxx.LAN-SMTP.com = ?
1.1.1.1
LAN
192.168.1.21 192.168.1.89
The LAN user's computer then sends traffic to IP address 1.1.1.1. NAT loopback uses the IP address
of the ZyWALL'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 130 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's LAN IP address and the ZyWALL 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.
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ZyWALL 110/310/1100 Series User's Guide