Unable to access some random websites : .gov, .mil, .orgs and .coms

For the last month or so I've been unable to access any .gov sites, .mil sites - some .orgs and .com sites as well. I can't find a pattern in what I can and cannot access. However, pages like facebook, and google/gmail work fine for me? This happens on Safari and Firefox - both of which are also the current versions. I'm currently using a macbook on Mountain Lion OSX 10.8.4, which I've recently updated (around the time I started having trouble accessing sites). I'm unable to locate the problem - can anyone help me out?