What's a "Network Protocol"?

If a bunch of networks are going to be connected, they have to agree on how to talk to one another. They agree on a common set of network "protocols" to exchange email, transfer files, even to emulate telephones.

A protocol is just a standardized way to talk.

When your phone rings, you pick it up and say, "Hello?"

I might respond with "Hi, this Brian. Is now a good time to talk about next week's meeting?"

And the conversation continues. When it's over, polite "telephone protocol" requires that we each say, "good bye", then hang up and close the connection.

Sending letters in the mail uses a couple of protocols. You remember writing letters, don't you?

You get out a piece of paper and write the date at the top of the page, then, if it's an official letter, you add your return address, the name and address of the recipient, and a polite salutation. "Dear Mr. Smith, I'm writing to tell you how pleased I am that you have joined our club..."

At the end of the letter, you close with "Sincerely," and your name, then fold it neatly so the recipient's name is visible, put it in a #10 envelope, and address the envelope in a format that your post office recommends so they can figure out how to deliver it. You put a stamp of sufficient denomination in the upper right corner on the face of the envelope and drop it in an official mailbox, and you're done. You don't worry about the details of how it gets picked up, the postage stamp gets canceled, the envelope is sorted, transported, sorted again, given to the correct carrier, and hand-delivered to the recipient's mail box.

That's all protocols, some of it simple courtesy, some required by or executed by your post office.

Email uses a conversation very similar to talking on the phone, the simple mail transfer protocol (SMTP). A computer with email to deliver to another computer connects to the receiving computer, which starts the conversation with "HELO". Literally, your mail server answers the phone and says. "HELO".

At the end of the conversation, after the mail has been transferred, the sending computer says, "QUIT", and the receiving computer (being a bit more polite), usually says "221 Bye". (The "221" is code for "transfer successful".)

That's all a protocol is: a polite conversation between computers.

But these are computers talking. They are fast. And they have a lot to say to each other, so your network stays pretty busy.