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What is the difference between HTTP and a URL as used for a website? How is that related to the layers of HTTP?

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  • They're completely dufferent things. Used for dufferent purposes. HTTP is a transfer protocol, URL is a resource locator. – TZHX Mar 24 '15 at 9:34
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A URL is an address used to locate documents. Examples of URLs are:

  • http://example.com/mypage.html
  • ftp://example.com/download.zip
  • mailto:user@example.com
  • file:///home/user/file.txt
  • tel:1-888-555-5555
  • http://example.com/resource?foo=bar#fragment

HTTP is the protocol used to fetch some URLs (the ones that start with http://) from servers. A simple HTTP request works like this:

  • Parse the server name out of the URL (example.com)
  • Look up the IP address for that server (93.184.216.34)

  • Open a socket to that server and write an HTTP request (ending with two new lines):

    GET /resource?foo=bar HTTP/1.1
Host: example.com

  • Wait for the response from the server:

    HTTP/1.1 200 OK
Accept-Ranges: bytes
Cache-Control: max-age=604800
Content-Type: text/html
Date: Tue, 24 Mar 2015 11:00:51 GMT
Etag: "359670651"
Expires: Tue, 31 Mar 2015 11:00:51 GMT
Last-Modified: Fri, 09 Aug 2013 23:54:35 GMT
Server: ECS (ewr/15BD)
X-Cache: HIT
x-ec-custom-error: 1
Content-Length: 1270

<!doctype html>
<html>
<head>
...

As you can see, a URL is an address and HTTP is a method of talking to a server.

I'm not sure what the HTTP layers you are referring to are.

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