Your average web application makes use of a lot of different protocols, most of them are quite old already.
It’s a web app, so you use HTTP, which originates back to 1989. Sending Mails? SMTP is part of the game too. As you are also probably using a domain, DNS is as well.
As modern web developers often we don’t know much about these protocols. I claim that this is a fatal mistake. There is so much to learn, let’s explore how these works, let’s discover their quirks and learn what modern applications can learn from them.