Uniform Resource Locators, URLs, are at the heart of the web's hypertext system. A URL contains all the information your browser needs to send a request to a server and tells the server what resources you're looking for. So what are the different elements of a URL and how does it work? URLs are divided into two essential parts, separated by a colon. Basic syntax of a URL is scheme:path. The scheme tells the browser which protocol it needs to use to retrieve a resource, and the path gives the location of the server and the resource. Protocols include things like HTTP, HTTPS and FTP, and email protocols like POP3, and IMAP. 

In this example, the scheme is HTTPS, and the path www.qa.com. The resource is web-development-fundamentals-html-and-css-qahtmlcss. Quite a bit of a mouthful, I know. Of course, it's pretty common not to type the scheme when we enter a URL into our browsers. We might just type the path. Or in our example, www.qa.com. This isn't actually a valid URL but these days, most browsers will just automatically enter the scheme into the syntax for you so that when you type an invalid URL into your browser, it'll still work as you expect. 

So that's the generic form of URL syntax but HTTP and HTTPS has its own specific syntax which is important because the web uses HTTP to retrieve resources. The HTTP URL syntax is Hostname:Port and Document-Path. The Hostname is always the name of the website you're accessing and while they often start with www, this is actually more of a convention than anything else. Hostnames can be followed by a colon and a port number. Common port numbers include 80, 443 and 8080. And if no port number or protocol is included, URLs will assume you're using HTTP and default to 80. Port numbers are important because they work as a kind of gateway that lets data in or rejects it. 

Firewalls work with open ports to let data pass through if they're sent to the right port. All data not directed to an open port is rejected. Port 80 is used to allow HTTP data to end, 443 is used to allow HTTPS data and 25 is used to allow email through. The document path is the location of the resource in the web service directory. If you leave the document path out of your URL, it will just default the home page of the site. Home pages are server and configuration dependent. And this means they could refer to a document path like index.html or just default.html. And that's it for this video. URLs have a specific kind of syntax that they must always follow if they're gonna work. Same is true of HTML-based URLs, which also have their own unique syntax.

Lectures

• Clients and Servers
• HTTP and its Interactions
• Client Requests in Details
• Server Response
• MIME and HTTP
• Security Issues and HTTPs