Why do shared nameservers undo IP diversity

Because a nameserver is a single public record, and one query returns it. Spread fifty sites across fifty unrelated address blocks, then point all fifty at one shared pair, and you have rebuilt exactly the grouping you paid to remove. The addresses differ. The DNS answer does not, and the DNS answer is easier to look up than the addresses are.

The mechanics of the leak

Every domain publishes which nameservers are authoritative for it. That information is not private, not incidental and not hidden behind anything. Anyone can ask a domain which nameservers serve it and get a definitive answer immediately.

If your fifty domains all answer with the same pair, that pair is a shared attribute across all fifty. Worse, it is a searchable one: given a nameserver, it is straightforward to enumerate the domains that use it. The address-level diversity you bought is still there and still real, and it has been rendered irrelevant by one record that groups everything in a single step.

Why it happens so often

Rarely from carelessness. Usually from one of two reasonable-sounding decisions:

• Leaving the default. The provider's shared pair works, nothing appears wrong, and nobody revisits it. The estate then shares a nameserver with the provider's entire customer base.
• Setting up one private pair for the estate. This feels like the correct move and is a genuine improvement over the default, but it still leaves fifty domains sharing one answer. It replaces the provider's grouping with your own.

The second is the more expensive mistake because it comes with the confidence of having done something about the problem.

The arrangement that actually works

A pair per domain, with the pairs pointed at different addresses. Fifty domains then produce fifty distinct answers, and there is no single record that returns more than one of your sites. Since private nameservers are included on every plan here rather than sold as an add-on, the cost of doing it properly is the setup time and nothing else.

Pointing every private pair at the same two addresses is the near-miss version: the names differ, the addresses behind them do not, and the grouping survives one step further down. Vary both.

Checking your own estate

1. Query a handful of your domains for their authoritative nameservers.
2. Compare the answers. Any repetition across supposedly unrelated sites is the problem described above.
3. Compare the addresses those nameservers resolve to, not only their names.

Do this on an estate you inherited before assuming it is fine. It is a five-minute check and it regularly finds that the expensive part of the arrangement was undone by the free part. Setup is covered in Nameservers and DNS, and the broader footprint picture in IP Diversity.