Chapter 13 NAT
ZyWALL/USG Series User’s Guide
393
13.3 NAT Technical Reference
Here is more detailed information about NAT on the ZyWALL/USG.
NAT Loopback
Suppose an NAT 1:1 rule maps a public IP address to the private IP address of a LAN SMTP e-mail
server to give WAN users access. NAT loopback allows other users to also use the rule’s original IP
to access the mail server.
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 271 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/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.
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
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