ZyXEL Communications SBG Series Support Notes page 24

multiple subnets on one VLAN or have one subnet spread across multiple
VLANs. Virtual LANs and IP subnets provide independent Layer 2 and Layer
3 constructs that map to one another and this correspondence is useful
during the network design process.
When do I need to use NAT?
a. Make local server accessible from the outside Internet.
When NAT is enabled the local computers are not accessible from
outside. You can use Multi-NAT to make an internal server accessible
from outside.
b. Support Non-NAT Friendly Applications
Some servers providing Internet applications such as some IRC servers
do not allow users to login using the same IP address. Thus, users on
the same network cannot login to the same server simultaneously.
What is NAT?
NAT (Network Address Translation-NAT RFC 1631) is the
translation of an Internet Protocol address used within one network to a
different IP address known within another network. One network is
designated as the inside network and the other is the outside. Typically, a
company maps its local inside network addresses to one or more global
outside IP addresses and "unmaps" the global IP addresses on the
incoming packets back into local IP addresses. The IP addresses for NAT
can be either fixed 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 them accessible to the outside world. If you do not
define any servers, the NAT offers the additional benefit of firewall
protection. In such case, all incoming connections to your network will be
filtered out by the CPE, thus preventing intruders from probing your
network.
For more information on IP address translation, please refer to RFC 1631,
The IP Network Address Translator (NAT).
How does NAT work?
According to the following figure, we define the local IP addresses as the
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