In: Categories » Electronics and communication » Protocols » NAT Operation
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NAT can be confused with a proxy server, but there are definite differences between the two. NAT is transparent to the source and destination computers, but a proxy server is not. The source computer has to be specifically configured to communicate with a proxy server, whereas the destination computer thinks that the proxy server is the source computer. Proxy servers usually operate at Layer 4 (the transport layer of the OSI Reference Model) or higher, and NAT operates at Layer 3 (the network layer). Because proxy servers are usually an add-on application, they might be slower than NAT, because NAT is accomplished in hardware. NAT is configured on the device you use to connect to an external network, whether it is a firewall, router, or computer. Before you get too far into the operation of NAT, you need to have a basic understanding of its many forms and the several ways in which it can be used:
One fact that might need to be mentioned at this point is that your internal network, or LAN, can often be referred to as a stub domain. When used in this manner, a stub domain is a LAN that uses IP addresses internally, with most of the network traffic having a local destination. Although you are allowed to have both registered and unregistered IP addresses in your stub domain, any network device that uses an unregistered IP addresses must use NAT to communicate with the outside world. One other benefit of implementing dynamic NAT on your device is that it can automatically create a simple firewall between your internal network and outside networks or the Internet. NAT does this by allowing only connections that originate inside your stub domain. This lets you limit a computer on an external network from reaching your computer unless your computer initiated the contact. Using static NAT allows you to define where a connection initiated by an external device can connect on your computers. For instance, you might want to connect an inside global address to a specific inside local address that is assigned to your web server. Keep in mind that this simple firewall should not be considered a replacement for items such as the Cisco Secure PIX Firewall or the Cisco IOS Firewall Feature Set, because TCP packets may be forged by an unauthorized user to gain access to your "protected" devices.
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