The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Essentially, the zone is the range of all records for the domain address, so when you open a URL inside a web browser, your laptop or computer asks the DNS servers worldwide where the domain address is hosted and from which servers the DNS records for the domain name should be retrieved. In this way a 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 website content is requested from the right location, a mail relay server detects which server takes care of the e-mails for the domain name (MX record) to ensure a message can be forwarded to the appropriate mailbox, and so on. Any modification of these sub-records is done using the company whose name servers are employed, so you can keep the website hosting and switch only your email provider for example. Each Internet domain has a minimum of two NS records - primary and secondary, that start with a prefix such as NS or DNS.