Why does the number of hosts increase and decrease so much?
The Hostcount collects its data via zone transfers. Many organisations are increasingly wary of allowing zone transfers of their domain for security reasons. It's also possible that some organsations wish to restrict zone transfers due to the increased load on their nameserver.
The impact of restricting zone transfers for a zone is that all the data about the zone and its child zones is lost.
The two primary reasons for major decreases in the number of hosts counted for a TLD can be identified as:
A provider, whose nameserver runs secondary for many of its customers, decides to restrict zone transfers, either via the nameserver config or an Access Control List (ACL), so that the machine doing the Hostcount loses access to hundreds or thousands of small zones at the same time, each of which has a few hosts.
- An end-user/provider with a very large zone decides to restrict transfers so that all of the hosts in the zone are lost to the Hostcount.
Major increases in the number of hosts occur when there is no restriction.