You can actually use two of these addresses (network & broadcast) - but you cannot really call it a proper /X block anymore. You will also have to make sure to

You can actually use two of these addresses (network & broadcast) - but you cannot really call it a proper /X block anymore. You will also have to make sure to disable the broadcast address you've used up on all of your hosts. This is all very fuzzy and some admins will look at you weird for doing this - but sometimes you've gotta do what you've gotta do. The third address (router) is actually used by your ISP's router's to reach your subnet.