NAT Full Form is Network Address Translation. NAT is a internet standard which is used for remapping of a single network or a group of networks into a single IP address. It efficiently enables a Local Area Network (LAN) to use a set of IP addresses for internal use of traffic and another set for the external use of traffic.
This methodology modifies network address information in the packet headers of IP datagram, so that these packet headers look like they are coming from the NAT gateway itself. And it further ensures that the return packets do not get blocked by the system firewall. NAT comes to the use heavily when the number of IP addresses provide by the ISP is less than the numbers of computer in use. In such situation, NAT is used so that every computer can gain access to the internet.
NAT is mainly concerned with source IP address and source TCP or UDP port. NAT can be useful in various ways like allowing a organization to use multiple IP addresses internally without conflicting with the external IP addresses, or allowing a organization to use multiple ISDN connections in a lone internet connection , or helping the connected network by providing it a sort of firewall and hides its internal IP address.