Network Address Translation(NAT) is the process of converting one Internet Protocol address to a different Internet Protocol address. Simply it is the process of mapping one IP address to another. Why we need NAT and map one I-P to another? The answer is a little bit big. Currently we adopts IPv4 to generate unique I-P addresses for all the computers connected to the Internet. But we know that today's increased popularity of Internet make it difficult to assign unique IP address to all the computers connected to Internet. Growth of bigger companies shows thousands of computers in a company is nothing and with in many networks thousands of thousand computers are available. Is it possible to provide unique address to all the computers in these kinds of large companies ? No, so the idea of NAT invented.
Using the concept of Network Address Translation (NAT) we can easily assign Internet Protocol address to all the computers present in the bigger companies by spending few actual addresses. Before explaining it we need to know the concept of private IP address and public IP address. Private IP address is a set of addresses which is not using for connecting to Internet. The ranges of private Internet Protocol addresses are described below.
10.0.0.0 to 10.255.255.255
172.16.0.0 to 172.31.255.255
192.168.0.0 to 192.168.255.255
The computers in companies and a LAN are addressed by using private addresses. But by using private IP addresses they cannot access Internet. So the company should purchase a set of public IP addresses and using NAT they will convert the private I-P address to the valid public Internet-Protocol address. So we can avoid the headache of assigning unique IP address to all computers present in the corporates. To implement it we are losing the private Internet Protocol addresses but we can use the same set of private IP addresses to the computers in a different corporate network. So NAT and the concept of private Internet-Protocol addresses helps us in better utilization of Internet Protocol address.
Types of NAT
The NAT is divided in to the following categories.
1. Static NAT
2. Dynamic NAT
3. Port Address Translation (PAT)
In static NAT one unregistered (private) IP address is mapped to one registered (public) Internet-Protocol address. This kind of mapping is predefined one.
In dynamic NAT a pool of public Internet Protocol addresses is shared by an entire private IP subnet. This mapping is not predefined but it is dynamic.In Port Address Translation (PAT)multiple unregistered Internet- Protocol addresses map to a single registered IP address by different TCP/UDP port numbers.