ZyXEL Communications ZyXEL Vantage VSG-1000 - Manual (Page 37)

VSG-1000 Vantage Service Gateway
System Configuration 4-3
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 4-1 NAT Definition
ITEM DESCRIPTION
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.
NAT never changes the IP address (either local or global) of an outside host.
4.7.2 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).
4.7.3 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.
4.7.4 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. 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.
For IPSec, the VSG does not support AH protocol.
In order to allow subscribers to establish multiple VPN connections to a remote VPN device with single-
connection-per-source limitation, set the VSG to perform NAT on the WAN. You need to configure NAT address