I remember how I learned networking protocols basically over telnet in the 90s. You could send email by telnet'ing to some SMTP server, using the (HELO, MAIL FROM, RCPT TO commands). I of course debugged my HTTP cgi gateway using telnet and sending "GET / HTTP/1.1\nHost:somehost.com\n\n". I connected to IRC using telnet, joining channels and sending privmsgs (although answering PING requests from the server become annoying quickly). I learned that you can connect to FTPs using telnet, and on wrongly configured ones (FXP), you could use the FTP control channel to have the content of an arbitrary ASCII file be sent over a socket - just to use it to have the FTP server connect to an IRC server and send message to my online buddies.

Good times. I look back at it with my rose-tinted glasses of course, but I wonder how it is today, given that everything is now SSL/TLS secured and you can basically not tamper with anything anymore without a deep understanding of the protocol and writing code first.