ZyXEL Communications USG FLEX H Series - Page 149

Chapter 9 NAT
USG FLEX H Series User’s Guide
149
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 106 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.
Figure 107 LAN to LAN Traffic
The LAN SMTP server replies to the Zyxel Device’s LAN IP address and the Zyxel Device changes the
source address to 1.1.1.1 before sending it to the LAN user. The return traffic’s source matches the
original destination address (1.1.1.1). If the SMTP server replied directly to the LAN user without the traffic
going through NAT, the source would not match the original destination address which would cause the
LAN user’s computer to shut down the session.
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
192.168.1.21
LAN
192.168.1.89
Source 192.168.1.89
SMTP
NAT
Source 192.168.1.1
SMTP