In general, all of the machines on the Internet can be categories as two types: servers and clients.

Servers are special computers that provide services to other machines. There are web servers, e-mail servers, FTP servers and so on. A server has a static IP address that does not change very often.

However, the computers or mobile devices we use everyday are clients. They send requests to servers and receives replies from them.

To make it happen, our devices connect to a special computer, called a router. Router allows multiple computers to join the same network. Router can be then connected to the other router, and the other router, and the other router, building this way a network of networks.