This is a DNS zone database that is created when you create primary forward lookup zone. It is used to resolve DNS names to IP addresses. Record A identifies the IP address associated with the host name. Record MX identifies the Mail Exchanger, a mail server installed within your domain. CNAME record or Canonical Name record identifies an alias for the computer. That is it whereas host can have several aliases, its IP can be the same. The service record or SRV identifies domain controllers. That is needed for the domain client computer to identify domain controller. When the client will find that the 'domain controller 0' is the domain controller for the 'active-directory-site-name._sites.dc'. This example is a dummy file. Notice _kerberos on the beginning of the string. This indicates that the Kerberos service will be used to log on client to domain. And the _tcp is identifier for the protocol (that is TCP protocol) used to work with the specified service. Those '600' throughout this file represent the TimeToLive (TTL) value for the record to stay active. Zero written right after SRV record identifies the client priority in case there are multiple records and can actually vary from 0 to 65535. Then goes weight that allows to balance load by checking the record weight. And then we see the 88th port used by Kerberos.