Chapter 7 Nat; Overview; What You Need To Know - ZyXEL Communications PMG5318-B20A User Manual

7.1 Overview

This chapter discusses how to configure NAT on the GPON Device.
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.

7.1.1 What You Need to Know

The following terms and concepts may help as you read through this chapter.
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.
NAT Definitions
Inside/outside denotes where a host is located relative to the GPON Device, 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 26 NAT Definitions
ITEM
Inside
Outside
PMG5318-B20A User's Guide
DESCRIPTION
This refers to the host on the LAN.
This refers to the host on the WAN.
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H A PT ER
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NAT
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