ZyXEL Communications 792H - Manual (Page 103)

Prestige 792H G.SHDSL Router
NAT 6-1
Chapter 6
Network Address Translation (NAT)
This chapter discusses how to configure NAT on the Prestige.
6.1 NAT Overview
NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a host in a
packet, for example, the source address of an outgoing packet, used within one network to a different IP
address known within another network.
6.1.1 NAT Definitions
Inside/outside denotes where a host is located relative to the Prestige, for example, the computers 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, for example, 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 traveling 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.
Table 6-1 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.
6.1.2 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