Chapter 15 Network Address Translation (Nat); Nat Overview; Table 15-1 Nat Definitions - ZyXEL Communications ZyWALL 5 User Manual

Network Address Translation (NAT)
This chapter discusses how to configure NAT on the ZyWALL. This chapter is only applicable

15.1 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 is changed
to a different IP address known within another network.
15.1.1 NAT Definitions
Inside/outside denotes where a host is located relative to the ZyWALL. 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.
TERM
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.
15.1.2 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 local
NAT

Table 15-1 NAT Definitions

DESCRIPTION
ZyWALL 5 Internet Security Appliance
Chapter 15
when the ZyWALL is in router mode.
15-1