Basics of NAT

Basics of NAT or Network Address Translation

Network address translation is a technique of remapping an IP address space into another by modifying network address information in the IP header of packets. It can be done when traffic is on move or in transit.

It permits private IP networks that use unregistered IP addresses to connect to the Internet.

It allows a single device, such as a router, to act as an agent between the Internet network and a local network or private network, which means that only a single unique IP address is required to represent an entire group of computers to anything external of their network.

The main purpose of its use is to limit the number of public IP addresses an organization must use, for both economy and security purposes. NAT also allows to connect to a TCP/IP network using a Token Ring adapter on the host machine.

NAT can be used to allow limited access based on selection to the outside of the network. Computers requiring special access outside the network can be assigned specific external IPs using NAT and allowing them to communicate with other computers and applications that require a specific public IP address. 

Types of NAT

Static NAT: When the local IP address is converted to a public IP address, the address remains same is called static NAT.

Dynamic NAT: In dynamic NAT, instead of choosing same IP address, the NAT chooses from a pool of public IP address. So, the device gets different IP address each time. 

Further NAT can be designed in a network through the below types-

• Full cone NAT
• Restricted cone NAT
• Port restricted cone NAT
• Symmetric NAT

Organization can also use a NAT gateway, for managed NAT service that provides better availability, higher bandwidth, and requires less administrative effort. 

Overloading - A special case of dynamic NAT that maps multiple unregistered IP addresses to a single registered (globally unique) IP address by using different port numbers. Dynamic NAT with overloading is also known also as PAT (Port Address Translation).

Overlapping - This occurs when your internal IP addresses belong to global IP address range that belong to another network. In such case, the internal IP addresses need to be hidden from the outside network to prevent duplication. NAT overlapping allows the use of internal global addresses by mapping them to globally unique IP addresses using static or dynamic NAT.

Advantages

• NAT allows several hosts be connected to Internet by using fewer globally unique IP addresses. This in turn results in conserving the scarce public IP addresses. The terms public / global is used in the sense that the IP addresses are globally unique and officially registered. 
• NAT supports load sharing on inside machines. The inside machines are accessed in a round robin fashion, thus sharing load.
• NAT offers some degree of security since IP addresses are not easily traceable. This is because, the actual host IP that is accessing the Internet is translated into outside IP address and vice versa. Thus, NAT offers protection against hacking.

Disadvantages

One disadvantage of NAT is that it increases delay. This is obvious since address translation is involved. Another disadvantage of NAT is that, when an application uses physical IP address, it may not function properly. This is because the physical IP address is changed by NAT.

-DR