The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Simply, the zone is the collection of all records for the domain, so when you open a URL within a browser, your laptop or computer asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain address must be retrieved. In this way a web browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an Internet protocol address and the site content is required from the correct location, a mail relay server detects which server manages the e-mails for the domain name (MX record) to ensure that a message can be delivered to the needed mailbox, and so forth. Any modification of these sub-records is done with the help of the company whose name servers are employed, enabling you to keep the website hosting and change only your email provider for instance. Each and every domain name has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.