ZyXEL Communications NXC5200 User Manual page 223

Chapter 14 NAT
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 92 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.
Figure 93 LAN to LAN Traffic
NAT
Source 192.168.1.1
Source 192.168.1.89
SMTP
SMTP
LAN
192.168.1.21 192.168.1.89
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
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NXC5200 User's Guide