Does your registrar point where you think it does? We ask the registry directly, then ask your nameservers, and tell you when they disagree.
Look up your domain's NS records and you are asking the zone what it thinks its nameservers are. That is not the question. A resolver that has never heard of your domain asks the registry, and goes wherever the registry points — which may still be the host you left last month.
So your zone can be perfect and your domain still broken, and every check you run says it is fine, because your own resolver cached the old answer and is happily using it. This page asks both sides and compares them.
Because that asks the zone, and the zone is not what resolvers follow. A resolver that has never heard of your domain asks the registry, and goes wherever the registry points. If the registry still points at your old host, your new zone is perfect and nobody is reading it. This page asks the registry directly.
The registry, for anyone who has not looked your domain up recently. That is why a mismatch can look fine to you and be broken for everybody else — your resolver has the old answer cached and keeps using it long after new visitors have stopped being sent to the same place.
Yes. Resolvers spread queries across every nameserver the registry lists, so a server that cannot answer is not skipped quietly — it costs a share of your visitors a timeout before they get an answer from somewhere else, and sometimes they give up first.
Usually not. A change you have just made is still reaching the other servers, which is how it is supposed to work, and it settles within minutes. If they stay different for hours, one has stopped receiving updates.
Import your zone from a BIND file, check it answers here before you switch the delegation, then point the registrar. Every zone is DDoS-protected, on every plan including the free one.
