As we know the internal networks use private IP calls (or reserved, non-routable). They are not valid on the Internet. IPs reserved for Internet are public (routable). A public IP example is 200234201101.
The Gateway for now we will use this generic name, but know that we can be talking about a router, access point , firewall , etc., it gets a valid IP on the Web because of its connection to the Internet access provider. It is as if the provider lend a valid IP to the router so that we can connect to the Internet.
But every computer on the network also has a private IP. How do they access the Internet? This is possible thanks to the NAT, which is a feature that must be enabled at the gateway.
The operation that performs NAT, basically, is to map that contained the internal IP and the local port of the computer. Thus, when that package out of the internal network to the Internet, it will have a valid IP (which is the IP gateway that took borrowed from the provider) and will carry a reference to this port and IP. When the package arrives at its destination, the computer receiving will know where to return the answer, because he knows the IP (which is recognized as a valid IP), and has the reference who sent the package.
On the other hand, if the target computer received this package declaring as sender private IP, used by the computer on the local network, it simply would not know where to send the response as private IPs are not used on the Internet and are treated as if did not exist.
Basically what the NAT does is pick up the packets coming from the internal network and leave them ready to be uploaded to the Internet, so it is properly received in the sender In addition, the sender will also have total conditions to send the correct answer .
Why do we need Port Forwarding?
As stated, this feature allows a remote computer to connect to a computer within the local network. This connection is achieved only because we associate advance, the router, communication ports and IP addresses. For example, we may associate the port 2133 to the IP 192.168.2.1. Thus, whenever an external computer "says" the router that he wants to communicate with the port in 2133, the router will "know" that he wants to communicate with the computer whose IP is 192.168.2.1.
For this scheme to work always, the computers on the network must use fixed IP, otherwise you will need to change the router settings to each new entry of computers to the network, which is not feasible.
Some important observations:
- The NAT must be enabled.
- The same port can not be redirected to more than one computer.
- You can assign multiple ports to a single IP address of the local network.
- Need to know what is the application port or service you want to offer.
Ho to configure Port Forwarding on routers?
There are various brands of routers and each case is different, so follow these steps:
- Access the site Port Forwarding
- You will find in alphabetical order the brand of your router, click on your.
- Then choose the model of your router.
- Now choose the service you want to do the redirection port (eg HTTP)
- Finally follow the steps that the site features.