What Nat Does; How Nat Works - E-data DSL-2120 Series User Manual

5.2.1.1 What NAT Does

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. You may also designate servers, such as a Web server and a telnet server, on your
local network and make them accessible to the outside world. With no servers defined, your
DSL-2120/2120W 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).
Inside/outside indicates where a host is located relative to the DSL-2120/2120W. The
computers hosts of your LAN are inside, while the Web servers on the Internet are outside.
Global/local indicates the IP address of a host in a packet as the packet traverses a
router. 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 of 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.
ITEM
Inside
Outside
Local
Global

5.2.1.2 How NAT Works

Each packet has two addresses – a source address and a destination address. For
Proprietary & Confidential
Revision: V1.0
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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Apr. 23, 2008