What Is Nat; How Nat Works - ZyXEL Communications P-661HNU-Fx Support Notes

P-661HNU Series Support Notes
and service including authentication, accounting, secure access and
configuration management.

10. What is NAT?

The 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, one company maps its local
inside network addresses to one or more global outsile 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 the IP address translation, please refer to RFC 1631,
The IP Network Address Translator (NAT).

11. How NAT works?

If we define the local IP addressed as the Internal Local Addresses (ILA) and
the global IP addresses as the Inside Gloable Address (IGA), see the following
figure. The term „inside‟ refers to the set of networks that are subject to
translation. The NAT operates by mapping the ILA to the IGA required for
communication with hosts on other networks. It replaces the original IP source
address (and TCP or UDP source port numbers) and then forwards each
packet to the Internet ISP, thus making them appear as if they came from the
NAT system itself (e.f., the PCE router). The CPE keeps track of the original
addresses and port numbers, so the incoming reply packets can have their
original values restored.
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