I'm going to hijack your top post to explain something. I'm a unix admin. I've been working on servers connected to the internet back before it was even called the internet. Most people don't realize how important DNS is to your security. If DNS becomes compromised your entire security plan can go to complete shit and your servers can be easily accessed. It would take too long to explain all the details but I can't stress enough that if someone even for a few hours compromised the DNS for Facebook etc then they very easily could have compromised Facebooks servers.

Simple version... imagine you have your database for an internal network shared across the whole building. You have multiple IPs and they might change so the security for that share is set to allow "blahblahblah.facebook.com" access. UNLIMITED ACCESS. You compromise the DNS and you tell it that your IP on a freaking DSL/Cable Modem in VA is blahblahblah.facebook.com. All the real people that should be accessing the data now can't cause they are just some rando IP with no name matched up in the DNS(because you deleted all the real records) but you match. So you go right through the firewall because it things you are a real Facebook IP and access the server.

The best part is as everything stops working all the traffic dies down so you have 80% of the bandwidth on a 10 zillion megabit line so your DL flies as such an insane speed that you're done downloading a multi-terabyte database in like an hour.

It also creates a lot of confusion because DNS is cached at different places at different times so the failures would happen randomly.

Bottom line if you know a little bit about how their security is setup and have a way to compromise DNS then you can walk right past even the best security setups like they are nothing. The only way to prevent this is to only us IP addresses which some people do but it can be a HUGE pain in the ass. Using names is often easier. You change to IP in the DNS and it changes EVERYTHING else for you. Problem is if a bad actor can change that DNS database they can own everything for a short while.