I remember long ago in my university times, you could simply, while sitting on a pc, simply write a message to another pc through the terminal. You just had to know his login name.

So as soon as he logs in, he could read the message.

Now, in my case, we don't have a login name, except when using bluetooth a name has to be defined. But ip is unique for each device.... Is it possible then to simply pass a message through the xterminal?