Chapter 12 Network Address Translation (NAT)

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.

Table 51

NAT Definitions

ITEM

 

DESCRIPTION

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.

 

 

 

NAT never changes the IP address (either local or global) of an outside host.

12.8.2 What NAT Does

In the simplest form, NAT changes the source IP address in a packet received from a subscriber (the inside local address) to another (the inside global address) before forwarding the packet to the WAN side. When the response comes back, NAT translates the destination address (the inside global address) back to the inside local address before forwarding it to the original inside host. Note that the IP address (either local or global) of an outside host is never changed.

The global IP addresses for the inside hosts can be either static or dynamically assigned by the ISP. In addition, you can designate servers, for example, a web server and a telnet server, on your local network and make them accessible to the outside world. If you do not define any servers (for Many-to-One and Many-to- Many Overload mapping), NAT offers the additional benefit of firewall protection. With no servers defined, your ZyXEL Device filters out all incoming inquiries, thus preventing intruders from probing your network. For more information on IP address translation, refer to RFC 1631, The IP Network Address Translator (NAT).

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