A master server can also be configured to notify slaves when a zone changes so that they can perform a zone transfer immediately, ensuring that they are always up to date.
Most of the time, it does not matter what the serial number is as long as it gets incremented.A server is typically responsible for providing information about the zones that it hosts, and for looking up information in other zones when requested to by DNS clients.For a zone hosted by a server to be available to DNS clients that do not query that server directly, it must be registered in the parent zone.Although there are several ways this can be done (such as reading the /etc/hosts file or querying an NIS server), DNS is the most common.As well as looking up IP addresses for hostnames, the DNS protocol can also be used to find the hostname associated with an IP address.Slave servers can request a copy of all the records in a zone at once by doing a zone transfer.
This is done a secondary DNS server when a zone is first added to it, and periodically when it detects that the zone has changed or the records in it have expired.
However, this does not have to be the case- a DNS server can be both a master for some zones and a slave for others.
There is no upper limit on the number of servers a zone can have, although few have more than three.
Other types include the NS or name server record which specifies the DNS server for the zone or a sub-domain, and the MX or mail server record type which defines a host that should receive mail for the zone.
Every zone should have at least one secondary server in case the primary is down or un-contactable for some reason.
However, by far its most common application is converting hostnames to IP addresses.