Active Directory is Microsoft technology that acts as a phonebook similar to DNS, however with AD, the focus is on identity and not network location.  Many 3rd party applications and network appliances have built in support for MS AD by using LDAP (Lightweight Directory Access Protocol) not to mention that Microsoft heavily uses the AD technology with all of their server lines.

DNS is one of the oldest and underutilized technologies out there.  Not only can DNS give you astetically pleasing website names such as questions.yourweb.com, comments.yourweb.com, forum.yourweb.com, but it can also help you get a administrative handle on your intranet as well.

Large corporations generally have a fully staffed Information Technology department with positions from Systems Architects to Site Technicians and the Management staff to support them.  However, most large corporations are not IT specialists and rely on their internal staff not only to support the current infrastructure, but address critical situations, train new employees, develop processes and procedures, AND research current/future technologies, develop deployment strategies, and move forward with deployment and further training.  This on top of a technology professional's need to keep themselves up to date on the latest client, corporate and back-end software solutions.