ZyXEL Communications USG40 User Manual page 268

Chapter 11 NAT
Figure 178 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/USG'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 179 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.
ZyWALL/USG Series User's Guide
268