Chapter 17 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 194 LAN Computer Queries a Public DNS Server

DNS

xxx.LAN-SMTP.com = ? 1.1.1.1

LAN

xxx.LAN-SMTP.com = 1.1.1.1

192.168.1.21192.168.1.89

The LAN user’s computer then sends traffic to IP address 1.1.1.1. NAT loopback uses the IP address of the ZyWALL’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 195 LAN to LAN Traffic

 

 

 

NAT

Source 192.168.1.1

Source 192.168.1.89

 

SMTP

 

 

SMTP

 

LAN

192.168.1.21192.168.1.89

The LAN SMTP server replies to the ZyWALL’s LAN IP address and the ZyWALL 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

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ZyWALL USG 50 User’s Guide