Chapter 12 NAT
NXC Series User’s Guide
194
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 103 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
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.
Figure 104 LAN to LAN Traffic
The LAN SMTP server replies to the NXC’s LAN IP address and the NXC 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,
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
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