General introduction about NAT

When two computers are on the same network (same subnet), these computers connect directly to each other, which means they can send and receive data directly with each other. If these computers are not on the same network layer and there is no direct connection, the data will be forwarded between these network layers and thus need a router (probably software or hardware). This is the case when a computer wants to connect to another computer on the internet. How does NAT work? NAT works as a router, its job is to forward packets between different network layers on a large network. You might also think that the Internet is a single network, but there are countless subnets. Routers are able to understand the different network layers around it and can forward packets to the right place.

Figure 1 NAT uses its own IP as a public IP for each client (client) with its own IP. When a client makes a connection or sends data to a computer on the internet, the data will be sent to the NAT, then NAT will replace the original IP address of that client and send the packet with NAT IP address. Remote computers or certain computers on the internet when receiving signals will send packets back to the NAT computer because they think that the NAT computer is the machine that sent the packets. NAT records the information table of computers that have sent outgoing packets on each service port and sends the received packets to the correct computer (client). The NAT performs the following tasks: Convert the source IP address to its own IP address, which means that the data received by the remote computer is the same as received from a computer with NAT configuration. Send data to the remote computer and remember which service port it used. Data received from the remote computer will be transferred to the client.

Does Figure 2 NAT work with any protocol and application? The protocol uses multiple connections or multimedia and multiple data types (such as FTP or RealAudio). With FTP, when you start the file transfer, you make a connection to the FTP server by the FTP client, the client connects to it and requests a file or directory transfer, with some FTP clients you will see a message. something like the port command, what this command line is doing is setting up a data connection to send the file or directory back to the FTP client. Doing so means that the client "tells" the server that "please connect to me on this IP address and on this port port to transfer data". The problem is that the client only tells the server to connect back on the internal IP address within its own LAN and thus the server will not find this IP address and fail if the server tries to search. and connecting to this address, this is the time to need NAT. Most NAT solutions (including WinGate) have special support for the FTP protocol and requirements for the computer to be configured. NAT is a computer that must have a static IP address.