Resolver Comparison
Whether three public resolvers agree yet about a domain.
Three public resolvers, run by three different operators. Not your own resolver: what your computer or your office network hands back is decided on your side of the connection and never reaches this server, so no page here can report it. Nor are these authoritative — they are caches, which is exactly why comparing them is worth anything.
Asking a resolver what it holds means telling it what you asked about. Comparing three answers requires asking three resolvers, so the name you type goes to all three. Privacy says the same in more detail.