When a client machine or a network node comes online, it automatically gets their names registered in DNS database.
Dynamic updates take place when there is a modification or change done at the client or when we have DHCP server.
There are 2 types of Dynamic updates
Useful when we do not want our DNS to maintain outside our network host information.i.e, client machines should be already registered in our domain controller.
DNS gets updated as and when what all the hosts come online get their names registered with DNS server.