Network Address Translation – Port Address Translation

                               NAT (Network Address Translation)

 Address translation substitutes the real address in a packet with a mapped address that is routable on the destination network. NAT is composed of two steps: the process by which a real address is translated into a mapped address and the process to undo translation for returning traffic.

Some of the benefits of NAT are as follows:

• You can use private addresses on your inside networks. Private addresses are not routable on the Internet.
• NAT hides the real addresses from other networks, so attackers cannot learn the real address of a host.
• You can resolve IP routing problems, such as overlapping addresses.

NAT Types

You can implement address translation as dynamic NAT, Port Address Translation (PAT), static NAT, or as a mix of these types.. The following translation types are available:

Static NAT:

In static NAT manual translation is performed by an address translation device, translating one IP address to a different one. If you have 100 devices, you need to create 100 static entries in the address translation table. Typically, static translation is done for inside resources that outside people want to access.

Dynamic NAT:

Dynamic NAT is mostly used when inside user needs to access outside resources. The global address assigned to the internal user isn’t important, since outside devices don’t directly connect to your internal users they just return traffic to them that the inside user requested.

Dynamic NAT is used when inside user wants to access external resource. When an inside user sends traffic through the address translation device, say a router, it examines the source IP address and compares it to the internal local address pool. If it finds a match, then it determines which inside global address pool it should use for the translation. It then dynamically picks an address in the global address pool that is not currently assigned to an inside device. The router adds this entry in its address translation table, the packet is translated, and the packet is then sent to the outside world. If no matching entry is found in the local address pool, the address is not translated and is forwarded to the outside world in its original state.

When returning traffic comes back into your network, the address translation device examines the destination IP addresses and checks them against the address translation table. Upon finding a matching entry, it converts the global inside address to the local inside address in the destination IP address field of the packet header and forwards the packet to the inside network. 

PAT (Port Address Translation):

With PAT, all devices that go through the address translation device have the same global IP address assigned to them, so the source TCP or UDP port numbers are used to differentiate the different connections. If two devices have the same source port number, the translation device changes one of them to ensure uniqueness.

Major difference between NAT and PAT is In NAT Only IP addresses are translated (not port numbers).