just one simple machine
This is one machine. Your application is on one machine.
connect to remote web-service
This is a second machine. The web service is on a second machine.
i need to what is the right tool for this diagram?
You don't need a tool. All you need is a drawing program. You can create the diagram with Paint. Windows systems come with Paint and it is free.
The diagram would have two boxes labeled "node 1" and "node" with a line connecting the boxes.