Check whether a website supports HTTP/2 and HTTP/3 (QUIC). The tool makes a real request and analyses the negotiated protocol.
The HTTP Version Check tool on 101IP makes a real HTTPS request to a website while attempting to negotiate HTTP/2 via ALPN. It also inspects the Alt-Svc header for HTTP/3 (QUIC) support and, when curl supports it, attempts a direct HTTP/3 connection.
HTTP/2 is negotiated during the TLS handshake via the ALPN (Application-Layer Protocol Negotiation) extension. If the server supports h2, it signals this to the client during TLS and the connection upgrades to HTTP/2. The response status line will start with HTTP/2.
HTTP/3 runs over QUIC (UDP). Servers advertise support via the Alt-Svc response header, for example: h3=":443"; ma=86400. If the server-side curl library supports HTTP/3, the tool also attempts a direct QUIC connection.