Network monitoring gives access to some information, but to get the true picture of how the time in an HTTP request is broken up, you need to get inside the browser.  In this presentation from the 2008 Velocity conference, Simon Perkins of Simtec Limited demonstrates the features of HTTPWatch and its strengths relative to network level monitoring. 

HTTPWatch is a commercial add-on for Internet Explorer that displays detailed HTTP performance and time charts.  Just press Record and see all HTTP requests to and from the server, grouped by page and broken down into stages from DNS lookup to completed download.  Specific benefits of monitoring within the browser are seeing browser cache effects and HTTPS traffic before it is encrypted.