Too many times have I seen websites that have DNS hosted on one box. Cpanel has been one of the main culprits with its highly dodgy way of mapping two ip addresses to one box and then having NS1.example.com and NS2.example.com.



The main problem with doing this is email. Just say your server goes offline, any email going to your server will be automatically dropped (and not stored in a mail queue) as the address cannot be found through DNS.



DNS Report a great site for checking all dns issues has recently been upgraded to detect this fault. Very useful.