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NAT has three main applications:
• If you have more inside hosts than you have outside IP addresses, the NAT service can translate multiple inside hosts to a single outside IP. The two most common scenarios for this are a typical Internet access router, where all the hosts on the inside are granted Internet access using very few—or even just one—outside IP address, or a modification of that example in which a lot of IPs are available, but not enough for our requirements. In both cases, the problem that NAT solves is the depletion of IP addresses; the fact is that very few registered IPs are available anymore, so being able to "reuse" them by NATing many hosts to a few of them is very helpful in extending the lifespan of the Internet address space.
• NAT can be used to solve two related and vexing network issues: The Overlapping Address Space and the Well-Meaning Admin Error. The Overlapping Address Space happens when we connect to another network that uses the same IP address range as we do; typically, this happens when we merge with another company. The problem is that we will have duplicate routes in different locations when the routers start updating each other, leading to instability, misrouting, and general mayhem.
The Well-Meaning Admin Error happens when the person responsible for the network design either fails to plan for future growth of his network, or simply makes a mistake because of ignorance or arrogance. This most often takes the form of a private network being addressed with public IPs that belong to someone else.
A real-world example of this occurred when a representative from the ISP told their customer to use the address space of 191.168.0.0 /24 for the inside network. This worked fine until Internet connectivity was required; at which time, it was pointed out that the 191.168.0.0 network was a registered Internet range belonging to an insurance firm in the Carolinas. This did cause some issues, but the problem was largely hidden by the NAT service, which translated all those inside local IPs to appropriate outside global addresses.
• NAT can also be used to give a whole cluster of machines (each with different inside local IPs) a single IP address that the clients can use. This is called Load Distribution, and works well for high-volume server clusters such as databases or web servers in which all the clients can use a single virtual IP to reach the service, and that single IP is NATed to all the real IPs of the physical servers.
Posted by steve10
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