ZyXEL Communications USG-100@USG-200 - V2.20 ED 2 - Page 430

Chapter 19 NAT
ZyWALL USG 100/200 Series User’s Guide
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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 320 LAN Computer Queries a Public DNS Server
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 321 LAN to LAN Traffic
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
192.168.1.21
xxx.LAN-SMTP.com = ?
LAN
DNS
192.168.1.89
xxx.LAN-SMTP.com = 1.1.1.1
1.1.1.1
192.168.1.21
LAN
192.168.1.89
Source 192.168.1.89
SMTP
NAT
Source 192.168.1.1
SMTP