For example, assume that the sub-domain en.wikipedia.org contains further sub-domains (such as something.en.wikipedia.org)
and that the authoritative name server for these lives at ns1.something.en.wikipedia.org.
A computer trying to resolve something.en.wikipedia.org will thus first have to resolve ns1.something.en.wikipedia.org.
Since ns1 is also under the something.en.wikipedia.org subdomain, resolving ns1.something.en.wikipedia.org requires
resolving something.en.wikipedia.org which is exactly the circular dependency mentioned above.
The dependency is broken by the glue record in the nameserver of en.wikipedia.org that provides
the IP address of ns1.something.en.wikipedia.org directly to the requestor, enabling it to bootstrap
the process by figuring out where ns1.something.en.wikipedia.org is located.