The most common of the NAT methods that is used in all the types of networks for connecting to the internet is the NAT overload. The way it functions and overcomes many limitations of the other types of NAT make it very useful for this field.
Characteristics of NAT Overload
1. Can be implemented everywhere:
– It does not matter whether you use any firewall application, a router or Microsoft internet or some other 3rd party application, NAT overload is employed for all of these for connecting home computers to the internet through just one single connection.
2. Used in different ways:
– There are other names by which NAT mode is known as NAT with port address translation (PAT), NAPT (network address port translation), IP masquerading etc.
– These different names have been derived from the different ways in which the NAT overload is used.
Working of NAT Overload
For understanding the working of the NAT overload it is important that you understand how UDP and TCP/ IP communications work.
– NAT overload is actually a combination of both dynamic and static NAT.
– It also contains some enhancements to achieve a more desired result.
– In NAT overload, either a dynamic or a static IP address is used which is mapped to the gateway’s public interface.
– Here, the gateway could be anything including firewall, router or a PC.
– All hosts lying within the boundaries of the private network are allowed to access the network services.
– Now you must be wondering how NAT overload achieves so much. Well the answer is through PAT i.e., port address translation.
– Consider having a host A on a private network with some IP address.
– It is transmitting some data to a specific IP address on the internet.
– Let us assume the receiving host to be a server and a default port is used for communication.
– When the data packets pass through the router, it changes their source IP address field.
– The ports remain unchanged.
– It is necessary to place the public IP address of the router in the place of source IP address of the data packet so that the server that the host was trying to connect to could know who is sending the request and could reply back.
– This is the reason for changing the source IP address of the packets.
– With all this, it is also ensured that the data packet that is being transmitted is able to track its way to the public interface of the router.
– After this, the router needs to have knowledge about the IP address of the host to which the reply is to be sent.
– Here, the ports come to the rescue. This sounds somewhat like the static NAT.
– What makes the NAT overload different from the static NAT is the presence of at least one more host.
– When two or more than two hosts are present on the private network, an equivalent number of IP addresses might be required for using static NAT i.e., one for each of the private host.
– One public IP address would be mapped to each of the hosts on the private network.
– This limitation is overcome by the NAT overload by making use of just one public IP address for all the hosts.
– But it utilizes thousands of ports that are available for identifying the session of each of the private host.
– It does not matter what type of IP address assignment is being used, the NAT overload is bound to work perfectly.
– NAT device is not affected by whether you have a permanent connection, a dynamic or static connection etc.
– This NAT mode helps a big deal in preserving the real public IP address.
– This is because only one IP address is required for allowing any number of hosts to access the internet.