NAT

NAT (network address translation)
In computer networking, network address translation (NAT, also known as network masquerading or IP-masquerading) is a technique in which the source and/or destination addresses of IP packets are rewritten as they pass through a router or firewall. It is most commonly used to enable multiple hosts on a private network to access the Internet using a single public IP address. According to specifications, routers should not act in this way, but it is a convenient and widely-used technique. There are the following NAT files.

Full cone NAT: NAT where all requests from the same internal IP address and port are mapped to the same external IP address and port. Furthermore, any external host can send a packet to the internal host, by sending a packet to the mapped external address. It is also known as "one-to-one NAT".
• Restricted cone NAT: NAT where all requests from the same internal IP address and port are mapped to the same external IP address and port. Unlike a full cone NAT , an external host (with IP address X) can send a packet to the internal host only if the internal host had previously sent a packet to IP address X
• Port restricted cone NAT: is like a restricted cone NAT, but the restriction includes port numbers . Specifically, an external host can send a packet , with source IP address X and source port P, to the internal host only if the internal host had previously sent a packet to IP address X and port P.
• Symmetric NAT: is a NAT where all requests from the same internal IP address and port to a specific destination IP address and port are mapped to the same external source IP address and port. If the same internal host sends a packet with the same source address and port to a different destination, a different mapping is used. Furthermore, only the external host that receives a packet can send a UDP packet back to the internal host.