HTML URL

### What is URL?
URL stands for Uniform Resource Locator is the global address of documents and other resources on the World Wide Web. Its main purpose is to identify the location of a document and other resources available on the internet, and specify the mechanism for accessing it through a web browser.

For instance, if you look at the address bar of your browser you will see:

```html
https://www.itshubao.com/html-tutorial/html-url.php
```
— This is the URL of the web page you are viewing right now.

### The URL Syntax
The general syntax of URLs is the following:

```html
scheme://host:port/path?query-string#fragment-id
```
A URL has a linear structure and normally consists of some of the following:

- **Scheme name** — The scheme identifies the protocol to be used to access the resource on the Internet. The scheme names followed by the three characters `://` (a colon and two slashes). The most commonly used protocols are `http://`, `https://`, `ftp://`, and `mailto://`.
- **Host name** — The host name identifies the host where resource is located. A hostname is a domain name assigned to a host computer. This is usually a combination of the host's local name with its parent domain's name. For example, `www.itshubao.com` consists of host's machine name `www` and the domain name `itshubao.com`.
- **Port Number** — Servers often deliver more than one type of service, so you must also tell the server what service is being requested. These requests are made by port number. Well-known port numbers for a service are normally omitted from the URL. For example, web service HTTP runs by default over port 80, HTTPS runs by default over port 443.
- **Path** — The path identifies the specific resource within the host that the user wants to access. For example, `/html/html-url.php`, `/news/technology/`, etc.
- **Query String** — The query string contains data to be passed to server-side scripts, running on the web server. For example, parameters for a search. The query string preceded by a question mark (`?`), is usually a string of name and value pairs separated by ampersand (`&`), for example, `?first_name=John&last_name=Corner`, `q=mobile+phone`, and so on.
- **Fragment identifier** — The fragment identifier, if present, specifies a location within the page. Browser may scroll to display that part of the page. The fragment identifier introduced by a hash character (`#`) is the optional last part of a URL for a document.

> Note: Scheme and host components of a URL are not case-sensitive, but path and query string are case-sensitive. Usually the whole URL is specified in lower case.