The default namespace that Socket.IO clients connect to by default is: /.
It is identified by io.sockets or simply io (docs).
This example copied from the documentation:
// the following two will emit to all the sockets connected to `/`
io.sockets.emit('hi', 'everyone');
io.emit('hi', 'everyone'); // short form
I assume it is the same for ‘on’, as it is for ’emit’: using ‘io.sockets’ is equivalent to using ‘io’ only, it’s just a shorter form.
To “namespace” your sockets, means assigning different endpoints or paths (which can be useful).
From an answer to this SO question:
“Socket.io does all the work for you as if it is two separate instances, but still limits the information to one connection, which is pretty smart.”