Port Address Translation (PAT)

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• Basic NAT supports multiple simultaneous connections but is still limited by the number of available public IP addresses
• smaller companies may only be allocated a single or small block of addresses by their ISPs
  • so a means for multiple private IP addresses to be mapped onto a single public address would be useful
• function provided by Port Address Translation (PAT)
  • aka Network Address Port Translation (NAPT), NAT overloading, one-to-many NAT, many-to-one NAT, or NAT masquerade
  • works by allocating each new connection to an ephemeral Transport layer port ID

Example: Port Address Translation (PAT)

• E.g., two hosts (10.0.0.101 and 10.0.0.102) initiate a web connection at the same time
  • the PAT service creates two new port mappings for these requests in its state table: 10.0.0.101:61101 and 10.0.0.102:61102
  • it then substitutes the private IP for the public IP and forwards the request to the public Internet
  • performs a reverse mapping on any traffic returned using those ports, inserting the original IP address and port number, and forwarding packets to the internal hosts