As a convenience, curl also allows users to leave out the scheme part from URLs. Then it guesses which protocol to use based on the first part of the host name. That guessing is basic, as it just checks if the first part of the host name matches one of a set of protocols, and assumes you meant to use that protocol. This heuristic is based on the fact that servers traditionally used to be named like that. The protocols that are detected this way are FTP, DICT, LDAP, IMAP, SMTP and POP3. Any other host name in a scheme-less URL will make curl default to HTTP.