Circular dependencies and glue records
Name servers in delegations appear listed by name, rather than by IP address.
This means that a resolving name server must issue another DNS request to find out the IP address
of the server to which it has been referred. Since this can introduce a circular dependency
if the nameserver referred to is under the domain that it is authoritative of, it is occasionally necessary
for the nameserver providing the delegation to also provide the IP address of the next nameserver. This record is called a glue record.