The Book

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===Public service on a private network===

Connecting to the Internet opens up a whole range of new and interesting possibilities for users. From "ordinary" surfing the Internet, receiving and sending e-mails through communication through various public online services and watching streaming content, to the ability to provide some of the services to the public. This does not necessarily mean opening access to the masses, but primarily launching one of the privately hosted services (web server, video surveillance server, private cloud server…) to the Internet so that we can access it from any public network. 

The procedure for this is very simple on most devices and is called Destination Network Address Translation (NAT). This means that the access network device (router), the packet that had its public address as the target address, replaces it with the address of the device on the private network, which is thus able to present its service on the Internet. If we have used the service of one of the dynamic DNS services and promptly notify him of changes in our public address, we will access our home web server via a permanent URL (Uniform Resource Locator) and not via an address that changes on a daily basis or more often.

We will quickly get used to the comfortable typing of a URL instead of an IP address and immediately encounter a problem that occurs when we want to use the same URL to access our published service from a private network and our favorite browser tells us that this is not possible. its actual private IP address, we will see that it is still available. 

Why is that so?