Ever thought about this? If I want to find the website example.com then my computer begins by contacting the TLD (.com) and finding the authoritative name server for example.com. If it is ns1.example.com, then how does my computer figure out the IP address for this subdomain? Is there a name server for the name server?
After searching about, I've been educated.
This problem is called circular referencing, for obvious reasons. So, to solve this problem, you register name servers with the same people you register the domain with. You specify the name of the name server and an IP address and they register what's called a glue record. This glue record is stored by the TLD (e.g., .com) name servers so that the IP address of your name server (e.g., ns1.example.com) can be sent when needed.
Bit 'o knowledge for you.
Need to check your glue records? A quick search on Google for glue record provides promising results, like this site.