Usefulness of the NAT server
The number of IP (Internet Protocol) on the Internet is limited. A router will reduce this number through NAT (Network Address Translation). The computers will have different addresses on the local network and will use a NAT gateway with the router, which has a unique IP address visible on the Web. This router makes it possible to hide the local IP addresses.
Consider two computers those have local addresses 192.168.1.10 and 192.168.1.11. They can connect to the Internet and they want their IP visible from the web, they will see they have the same, eg 77.56.14.143.
Through this process, an IP address visible on the Internet can correspond to several computers that will use a router and a NAT server.
Port Forwarding and NAT Server
The main drawback is the following: Imagine a software peer-to-peer (P2P) wants to send packets (a file you download). The principle is to specify the port in the software where the data will need to head. How, when it will happen on the public IP, know which computer on the LAN should send packages from? We know the port but not the exact address on the LAN. To solve this, the Port Forwarding (redirection / port translation) is used. This will allow you to indicate which computer (with its local address) must be sent a package that arrived on a certain port. So if you have two PCs that use the same P2P software, it will take two to make two different ports and redirects.
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To summarize
- The router through the NAT server is the intermediary between local addresses and IP visible from the Internet.
- It reduces the number of addresses on the Web (one BS is associated with multiple PCs connected to the LAN).
- Port forwarding allows the use of software that must receive data on a specific port.
