ZyXEL Communications zywall 50 User Manual

ZyWALL 50 Internet Security Gateway 

NAT   6-1 

Chapter 6  

Network Address Translation (NAT) 

NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a host in a 
packet, e.g., the source address of an outgoing packet, used within one network to a different IP address 
known within another network.  

Inside/outside denotes where a host is located relative to the ZyWALL, e.g., the workstations of your 
subscribers are the inside hosts, while the web servers on the Internet are the outside hosts.  

Global/local denotes the IP address of a host in a packet as the packet traverses a router, e.g., the local 
address refers to the IP address of a host when the packet is in the local network, while the global address 
refers to the IP address of the host when the same packet is travelling in the WAN side.  

Note that inside/outside refers to the location of a host, while global/local refers to the IP address of a host 
used in a packet.  Thus, an inside local address (ILA) is the IP address of an inside host in a packet when 
the packet is still in the local network, while an inside global address (IGA) is the IP address of the same 
inside host when the packet is on the WAN side. The following table summarizes this information. 

Inside 

This refers to the host on the LAN. 

Outside 

This refers to the host on the WAN. 

Local 

This refers to the packet address (source or destination) as the packet travels on the LAN. 

Global 

This refers to the packet address (source or destination) as the packet travels on the 
WAN