How many DNS servers are enough? Using the Internet and any other sources of information, come up with some guidelines...

How many DNS servers are enough? Using the Internet and any other sources of information, come up with some guidelines on how many DNS servers you would install on small, medium, or enterprise networks. Allow for variances in your guidelines based on requirements and geography. For example, a network with availability requirement of 99.9999% over five geographic locations would require additional redundancy. The ideas in your guidelines should be your own