Chapter 6; Nat Overview - Telkom ADSL 5100 Instruction Manual

Network Address Translation (NAT)
This chapter discusses how to configure NAT on the TELKOM ADSL 5100.

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 TELKOM ADSL 5100, 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.
ITEM
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.
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

Chapter 6

DESCRIPTION
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