What Nat Does; How Nat Works; Vpn And Nat - ZyXEL Communications Vantage Service Gateway VSG-1200 V2 User Manual

VSG-1200 V2 User's Guide

8.1.2 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 network and make them accessible to the outside world. If you do not define any
servers, NAT offers the additional benefit of firewall protection. With no servers defined, your
VSG 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).

8.1.3 How NAT Works

Each packet has two addresses – a source address and a destination address. For outgoing
packets, the ILA (Inside Local Address) is the source address on the LAN, and the IGA (Inside
Global Address) is the source address on the WAN. For incoming packets, the ILA is the
destination address on the LAN, and the IGA is the destination address on the WAN. NAT
maps private (local) IP addresses to globally unique ones required for communication with
hosts on other networks. It replaces the original IP source address in each packet and then
forwards it to the Internet. The VSG keeps track of the original addresses and port numbers so
incoming reply packets can have their original values restored.

8.1.4 VPN and NAT

A VPN (Virtual Private Network) provides secure communications between sites without the
expense of leased site-to-site lines. A secure VPN is a combination of tunneling, encryption,
authentication, access control and auditing technologies/services used to transport traffic over
the Internet or any insecure network that uses the TCP/IP protocol suite for communication.
The VSG allows subscribers to create a VPN tunnel to a remote site.
Note: For IPSec, the VSG does not support AH protocol.
By default, the VSG performs NAT on the LAN; mapping multiple private LAN addresses to a
single public address on the WAN. This prevents subscribers from creating multiple VPN
connections to a remote VPN device that allows only one VPN connection per source IP
address.
75 Chapter 8 NAT Pool