ZyXEL Communications Prestige 861H-G1, User Manual

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Prestige 861H User’s Guide
Network Address Translation (NAT) Screens 8-1
Chapter 8
Network Address Translation (NAT)
Screens
This chapter discusses how to configure NAT on the Prestige .
8.1 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.
8.1.1 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 8-1 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.
8.1.2 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 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