Like others said, every corporate windows network uses windows domain controllers as primary DNS resolvers. I wanted to add that, as computers are added to AD,

Like others said, every corporate windows network uses windows domain controllers as primary DNS resolvers. I wanted to add that, as computers are added to AD, you get a PTR record automatically in windows. If you have even a medium size network where new computers get added all the time, it would be a pain to autocreate their PTR records in bind. Also, for forwarders, I've seen DC's do full on recursive resolution themselves but when you need a forwarder you would ideally use a commercial solution that probably uses bind internally instead of rolling your own Linux bind server(typically)