Edit in JSFiddle

var range = document.createRange();
var p1 = document.getElementById("p1");

var helloNode = p1.firstChild.firstChild; // b > TEXT_NODE : Hello
var worldNode = p1.lastChild; // TEXT_NODE :  world

range.setStart(helloNode, 2); 
range.setEnd(worldNode, 3); 
document.write(range.toString());
console.log(range);

<p id="p1"><b>Hello</b> world!</p>