Nat Definitions; What Nat Does - ZyXEL Communications VMG1312-B Series User Manual

11.9.1 NAT Definitions

Inside/outside denotes where a host is located relative to the 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 65 NAT Definitions
ITEM
Inside
Outside
Local
Global
NAT never changes the IP address (either local or global) of an outside host.

11.9.2 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 (for Many-
to-One and Many-to-Many Overload mapping), NAT offers the additional benefit of firewall
protection. With no servers defined, your 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).
Chapter 11 Network Address Translation (NAT)
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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