gateway

• The purpose of NAT protocol is to reduce usage of public IPs
• A host needs a public IP to connect to Internet
• If the host is part of a LAN with a gateway router, a host can use private IP to make requests to public Internet.
• The public Internet would see that all the requests are originating from a LAN (i.e. the gateway router)
• A router has a local LAN IP and a public IP.
• The request flows as following:
  • A local host in the LAN can make a request to a web server on Internet.
  • The host request goes frpm local host IP and port to the local gateway.
  • The gateway maintains a NAT table.
    • An entry in the NAT table will have the source and destination mapping

      --------------------------------------------------------------------
      Local Host IP | Local Host port | Gateway public IP | Gateway port |
      --------------------------------------------------------------------
      

      • The gateway creates a port that maps request to and from the local host to the public Internet web server.
      • Hence the public server would always see the gateway IP and port and LAN host would be anonymous.
    • NAT is essentially a kind of multiplexing local hosts requests over gateways single IP and multiple ports, assigned to each local host.

Reference

• https://www.ionos.com/digitalguide/server/know-how/nat-how-network-address-translation-works/

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