As discussed in the DNS overview, each authoritative DNS server is only authoritative over a small part of the DNS tree, its zone. So if you’re in charge of a zone, and you want to delegate authority for a subzone to another DNS server, how do you do that?

Consider this simple example: say we’re authoritative for the acdam.net. zone, and we want to delegate authority over the europe.acdam.net. subzone to the guys in Europe. We do this by adding the following (the so-called glue records) to our zone:

• NS Resource Records for each of the DNS servers authoritative for the subzone
• A or AAAA records for the DNS servers that are in the subzone itself1)

So what we see is that the NS records for a zone are actually stored in two places: the zone itself, and the parent zone. You should always make sure the two are in sync; those not being in sync is a common cause for domain names to be unresolvable.

In our case, the guys in Europe have their own DNS server, ns1.europe.acdam.net., and the other DNS server is in the acdam.com. domain. So we’d need to add the following resource records to the acdam.net. zone:

europe       4h  NS  ns1.europe
ns1.europe   4h  A   
europe       4h  NS  ns2.acdam.com.