A friend of mine mentioned recently that he has to work out subnet masks in his head for an exam and commented in reality he’d just use a subnet calculator. Whilst this is probably true, there’s a quick trick that might help if you’re calculating subnets under duress. This isn’t a full write up and offers no real explanation of why it works, it’s just pointing out a trick you may have missed which might come in handy one day!
There are a couple of sites out there which will take a look at the configuration of your site and give pointers as to where you can tighten up your configuration, pointing out if you’re missing headers such as Content-Security-Policy, X-Frame-Options or X-XSS-Protection.
If you run a WordPress Blog there’s a quick way of adding and removing headers – you can do it within the WordPress Admin interface, with the Appearance Editor: