Network Address Translation (Nat); Introduction; Nat Definitions; What Nat Does - ZyXEL Communications PRESTIGE 1400 User Manual

8.1

Introduction

NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a host in a packet,
e.g., 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, e.g., the workstations 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 across a router, e.g., 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 travelling 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.
The IP address (either local or global) of an outside host is never changed.
8.1.2

What NAT Does

In its 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 the inside local address before
forwarding it to the original inside host.
The global IP addresses for the inside hosts can be either static or dynamically assigned by the ISP. In addition, you
can designate servers, e.g., 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 (for Many-to-One and Many-to-Many Overload mapping - see
next), NAT offers the additional benefit of firewall protection. If no server is defined in these cases, all incoming
inquiries will be filtered out by your Prestige, thus preventing intruders from probing your network. For more
information on IP address translation, refer to RFC 1631, The IP Network Address Translator (NAT).
NAT

Network Address Translation (NAT)

Prestige 1400 WAN Router with Integrated Ethernet Switch
This chapter discusses how to configure NAT on the Prestige.
DEFINITION
Chapter 8
8-1