ZyXEL Communications ZYWALL USG 20 Manual page 344

Chapter 17 NAT
For example, a LAN user's computer at IP address 192.168.1.89 queries a public
DNS server to resolve the SMTP server's domain name (xxx.LAN-SMTP.com in this
example) and gets the SMTP server's mapped public IP address of 1.1.1.1.
Figure 205 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 206 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
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ZyWALL USG 20/20W User's Guide