ZyXEL Communications UAG5100 - 14.3 NAT Technical Reference

Chapter 14 NAT
UAG Series User’s Guide
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14.3 NAT Technical Reference
Here is more detailed information about NAT on the UAG.
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 172.16.0.89 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 143 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 UAG’s lan1 interface (172.16.0.1) as the source address of the traffic going from the LAN
users to the LAN SMTP server.
172.16.0.21
xxx.LAN-SMTP.com = ?
LAN
DNS
172.16.0.89
xxx.LAN-SMTP.com = 1.1.1.1
1.1.1.1