DNS info

Also we have about 4000 machines. in 4 different buildings. How many DNS servers would you recommend?

You might be interested the Video tutorial which is easy to understand.

http://www.youtube.com/watch?v=8crmwK-hM6M
http://www.youtube.com/watch?v=2qWpjU4Tiss

Tutorial online.

http://www.verisigninc.com/en_US/domain-names/online/how-dns-works/index.xhtml

A dns actually convert IP to a name.

it either internal or external.

internal - use when you join a domain. it doesn't matter how many machine or building you have as long as you dns server connect to those PC. the domain design is depend on how you want it to be design..

external - you need a dns in order for your url to work. you can use one dns for as many machine you want.. example google dns 8.8.8.8

I'll be blunt. You won't learn the ins and outs of DNS from a tutorial. Or even everything you need to know. Heck, my usual response to broad questions is "buy a book so an author gets paid for his knowledge" ...and while that still applies here, you won't even learn everything from a book. The topic is too broad. Once you learn DNS basics, you want to learn about zone transfers. Or how Active Directory relies on DNS. Or DNSSec. Or DNS on IPv6. As a back one internet protocol, DNS is always evolving and being updated with new RFCs to meet evolving technology. You can't just spend a few hours and learn the "ins and outs."

Now with that said, a starting place (as good as any) is the O'Reilly book "DNS and BIND." You may never use BIND, but understanding how it works and reading its flat text zone files still helps from an educational standpoint. And from there you'll at least have a better idea of "next topics" you'll want to focus on in your environment.