Nat Technical Reference - ZyXEL Communications UAG4100 User Manual

15.3 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 110 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.
Chapter 15 NAT
xxx.LAN-SMTP.com = ?
1.1.1.1
LAN
172.16.0.21
UAG4100 User's Guide
DNS
xxx.LAN-SMTP.com = 1.1.1.1
172.16.0.89
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