I'm looking into getting our company set up to use a Domain instead of the simple work-group we've been using for, well, forever. We have 3 locations each with it's own dhcp server and subnet with a 3meg VPN connection from each remote location feeding a 6meg VPN connection to the main location. Based on that how many Domain controllers do we need? My original thought was 4, one per remote location and two for the main office but I'm not sure if I shouldn't have 2 per location as backup for each site. Can a domain server be used over subnets? Originally I thought that I'd use a domain controller in the main office as a primary and then delegate the remaining 3 as backups so a user would normally log into the main office controller unless the connection between sites was down or if the main one was down the backup at the office could be used.
Any suggestions for reading material on this or a website I can get some more info on Domain controller topography?