What is a URL?

    In the last section I showed you how to construct a Uri object. I also explained that a UriFormatException would be thrown if an invalid URL is specified. To understand what an invalid URL is, you should first understand the format of a good URL. URLs follow the following format:

Scheme://Host/Path?Query

    As seen above, the URL is made up of the following four components:

• Scheme
• Host
• Path
• Query

    In the next sections, each of these components will be discussed. We will start with the scheme.

URL Scheme

    The scheme is the protocol that will be used to transfer data. This book will explain the "http" and "https" schemes. Many of the more common schemes are listed in Table 2.1.

Table 3.1: Common HTML Schemes

    Following the URL scheme, the URL’s host is specified. The URL host will be explained in the next section.

URL Host

    The host specifies to which server the HTTP request is to be directed. There are several different formats in which the URL host can be represented. First, it can be in the typical domain form, such as:

www.httprecipes.com

    Second, it can be expressed as an IP address, such as:

127.0.0.1

    Finally, it can be expressed as a symbol that is resolved in the “localhost” file on the computer, such as:

localhost

    Following the URL host is the URL path and query. The URL path and query will be discussed in the next section.

URL Path and Query

    The path specifies which file to retrieve, or which script to run on the server. The "query" specifies parameters to be passed to the URL. The query immediately follows the path delimited by a question mark. The following URL specifies only a path:

http://www.httprecipes.com/1/1/cities.php

    The above URL specifies a path of “/1/1/cities.php”.

    Parameters can be passed using the query portion of the URL. The following URL demonstrates this concept.

http://www.httprecipes.com/1/1/city.php?city=2

    The above URL passes one parameter using the query string. A parameter named “city” is passed to the query string. The parameter “city” has the value of “2”. It is also possible to pass multiple parameters. If you are passing multiple parameters, separate them with the ampersand symbol (&). The following URL makes use of the ampersand to pass in two parameters. These parameters are named “city” and “zip code”.

http://www.httprecipes.com/1/1/city.php?city=2&zipcode=63017

    So far the URLs we have examined all contain standard ASCII (American Standard Code for Information Interchange) letters and numbers. In the next section, you will learn how to encode special characters into the URL.