PuTTY can use the Rlogin protocol to connect to a server.
Rlogin was similar to Telnet in concept, but more focused on connections between Unix machines. It supported a feature for passwordless login, based on use of ‘privileged ports’ (ports with numbers below 1024, which Unix traditionally does not allow users other than root to allocate). Ultimately, based on the server trusting that the client's IP address was owned by the Unix machine it claimed to be, and that that machine would guard its privileged ports appropriately.
Like Telnet, Rlogin runs over an unsecured network connection.