Technical Reference - ZyXEL Communications NXC2500 User Manual

Chapter 11 NAT

11.3 Technical Reference

The following section contains additional technical information about the features described in this
chapter.
NAT Loopback
Suppose a 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 79 LAN Computer Queries a Public DNS Server
DNS
xxx.LAN-SMTP.com = 1.1.1.1
xxx.LAN-SMTP.com = ?
1.1.1.1
LAN
192.168.1.21 192.168.1.89
The LAN user's computer then sends traffic to IP address 1.1.1.1. NAT loopback uses the IP address
of the NXC's LAN interface (192.168.1.1) as the source address of the traffic going from the LAN
users to the LAN SMTP server.
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