Subdelegation is the process of assigning a separate zone file and authority to a child node of a parent zone. In many cases, where a domain may not be owned by a given entity, but control over a certain part of that name is being granted, subdelegation is the answer. This is applicable to both forward and reverse DNS zones.
Subdelegation of a Forward DNS zone
Quite simply, this involves creating NS records on a certain hostname to tell the DNS resolver where to look for a zone file.
Let’s say those enterprising fellows over at example.com decided to embark on an experimental project where they happened to be running their own DNS for a certain resource. Knowing full well that everything in this subdomain would be handled by that authoritative server, and not NS1, they went into their account and put NS records in place where they would otherwise have put an A, AAAA or CNAME record.
Upon successful record creation, and saving the changes, queries for newproject.example.com are now being handled by the server specified in the NS record for that name.
Subdelegation of a Reverse DNS zone
Reverse DNS, for the uninitiated, is exactly what it sounds like it is. Instead of looking for a number assigned to a name, it’s looking for a name that correlates to a given IP address. This is achieved by using pointer records (PTR) that reference the infrastructure top-level domain, arpa. Now, because ‘arpa’ is not something one can just go buy a piece of like com or net, any and all reverse zones are a subdelegation of that top level domain.
More information on setting up Reverse DNS can be found in the NS1 Knowledgebase.