ZyXEL Communications Broadband Security Gateway P-312, User Manual, Page: 74 / 254

P312 Broadband Security Gateway
NAT 6-1
Chapter 6
Network Address Translation (NAT)
This chapter discusses how to configure NAT on the Prestige.
6.1 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.
6.1.1 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.
Table 6-1 NAT Definitions
Term Definition
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.
Please note that the IP address (either local or global) of an outside host is never
changed.
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 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. 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, e.g., a web server and a telnet server, on your local network and make