Technical Reference; Nat Definitions; What Nat Does; How Nat Works - ZyXEL Communications P-660HNU-F1 User Manual

Chapter 11 Network Address Translation (NAT)

11.5 Technical Reference

This section provides some technical background information about the topics covered in this
chapter.

11.5.1 NAT Definitions

Inside/outside denotes where a host is located relative to the ZyXEL 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 51 NAT Definitions
ITEM
Inside
Outside
Local
Global
NAT never changes the IP address (either local or global) of an outside host.

11.5.2 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, 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).

11.5.3 How NAT Works

Each packet has two addresses – a source address and a destination address. For outgoing packets,
the ILA (Inside Local Address) is the source address on the LAN, and the IGA (Inside Global
180
DESCRIPTION
This refers to the host on the LAN.
This refers to the host on the WAN.
This refers to the packet address (source or destination) as the packet travels on the LAN.
This refers to the packet address (source or destination) as the packet travels on the WAN.
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