NS Records identify DNS servers responsible (authoritative) for a Zone. A Zone should contain one NS Record for each of its own DNS servers (primary and secondary). This mostly is used for Zone Transfer purposes (notify). These NS Records have the same name as the Zone in which they are located.
But the most important function of the NS Record is Delegation. Delegation means that part of a domain is delegated to other DNS servers.
You can also delegate sub-domains of your own domain name (such as subdomain.yourdomainname.com) to other DNS servers. An NS Record identifies the name of a DNS server, not the IP Address. Because of this, it is important that an A Record for the referenced DNS server exists, otherwise there may not be any way to find that DNS server and communicate with it.
If a NS Record delegates a sub-domain (subdomain.yourdomainname.com) to a DNS Server with a name in that sub-domain (ns1.subdomain.yourdomainname.com), an A Record for that server (ns1.subdomain.yourdomainname.com) must exist in the Parent Zone (yourdomainname.com). This A Record is referred to as a Glue Record, because it doesn't really belong in the Parent Zone, but is necessary to locate the DNS Server for the delegated sub-domain.
There you would find 3 fields:
- Zone: If you wish to create a Name Server for a sub-domain like ns1.subdomain.yourdomainname.com, then enter the Zone as subdomain.yourdomainname.com in this text box.
- Value: This is the Name of the Name Server to be created or another Fully Qualified Domain Name that you want to make responsible for this Zone. Thus, if you wish to create an NS Record for subdomain.yourdomainname.com, you may enter the value as ns1.subdomain.yourdomainname.com. Or, you may want a Name Server like ns1.subdomain.yourdomainname.com to be delegated to another Fully Qualified Domain Name like dns1.anyotherns.com.
- TTL: This is the Time To Live for this Record. Any Server which once queries this Record will query it again after this time interval. The ideal TTL is 86400, which is 1 day. It can not be set to less than 14400, i.e. 4 hours