What is an RFC?
Check 'er out.
-->For ISP's who are not accredited Domain Registars: In Canada and probably all of North America, you are required two seperate DNS servers to "fully" host a domain. Which is usually what an ISP does.
ns1.ispdomain.com
ns2.ispdomain.com
Or
ns1.ispdomain.com
ns2.another_host_with_dns_records_for_that_domain_as_backup/redundancy.com
-->For personal use and you want to host your own domain on your own network with a static IP or via a Dynamic DNS service:
ns1.yourdomain.com
ns2.you_isp_who_has_your_DNS_records.com
Would be the best setup.
In Canada at least, the ns (or DNS servers) do not have to be in different subnets or physical locations.
For example:
ns1.ispdomain.com = 10.80.1.1
ns2.ispdomain.com = 10.80.1.2
Hope this helps!