Chapter 12 NAT
ZyWALL ATP Series User’s Guide
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12.3 NAT Technical Reference
Here is more detailed information about NAT on the Zyxel Device.
NAT Loopback
Suppose an NAT 1:1 rule maps a public IP address to the private IP address of a LAN SMTP email 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 229
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
Zyxel Device’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