URI vs URL: Differences, Similarities, and Uses

URIs (Uniform Resource Identifiers) and URLs (Uniform Resource Locators) are related but distinct concepts that are important to understand, particularly when developing your website or application. For example, the REST (Representational State Transfer) API, which is widely used for building site architecture, relies heavily on the proper use of these resource identifiers.

Today, we'll go over the differences between URIs and URLs, as well as explain their components, uses, relation with one another, and some best practices for using them.

What is a URI?

A URI, or Uniform Resource Identifier, is an overarching term describing a string of characters used to identify or name a resource on the internet or any other system. It provides a standardized way to locate and access various resources, be it websites, documents, images, or even physical objects in the context of the Internet of Things (IoT).

Syntax of a URI

A URI typically consists of several components, collectively referred to as the syntax, that work together to identify a resource:

Scheme: This specifies the protocol or method to access the resource, such as http://, https://, ftp://, or mailto:. The scheme informs the system how to interpret and process the remaining parts of the URI.

Authority: This part includes the domain name or IP address where the resource is located. It provides information about the host server or system that houses the resource.

Path: It represents the hierarchical structure or location of the resource within the host. The path segments are separated by forward slashes (/) and often resemble a file system structure.

Query: This optional component allows for passing parameters or data to the resource. It starts with a question mark (?) and consists of key-value pairs separated by an equals sign (=) and ampersands (&).

Fragment: Also optional, it points to a specific portion or section within the resource. The fragment is separated from the rest of the URI by a hash symbol (#) and is often used for internal navigation within a web page or document.

Let's use an example URI to breakdown each component of the syntax:

http://www.example.com/products/category1/item?color=green&size=large#details

In this URI:

• "http" is the scheme

• "www.example.com" is the authority

• "/products/category1/item" is the path

• "color=green&size=large" is the query

• "details" is the fragment

Types of URIs

There are two main types of URIs:

URLs (Uniform Resource Locators)

A URL is a specific type of URI that provides the location or address of a resource on the internet. It also uses the scheme, authority, and path components for accessing and retrieving resources.

URNs (Uniform Resource Names)

A URN is a persistent, location-independent URI used to uniquely identify a resource. Unlike a URL, it does not specify the location of the resource, but rather provides a name or identifier that can be resolved to find its location.

What is a URL?

A URL, or Uniform Resource Locator, is a specific type of URI used for locating and accessing resources on the internet. It serves as an identifier and pathway to retrieve and interact with online resources like web pages, files, and images.

Components of a URL

Like URIs, URL syntax consisting of several components:

Scheme: This specifies the protocol for accessing the resource, such as http:// or https://.

Authority: This part includes the domain name or IP address of the web server hosting the resource.

Path: It represents the hierarchical location of the resource within the web server.

Query: This optional component allows passing parameters or data to the resource.

Fragment: Also optional, it points to a specific section within the resource for internal navigation.

Using URLs to Locate Web Resources

When you enter a URL into a web browser or click a hyperlink, the browser uses the information in the URL to establish a connection with the appropriate web server and request the specified resource.

The scheme determines the protocol (e.g., http or https), while the authority identifies the hosting web server. The path navigates to the resource's location, and the query and fragment provide additional information or parameters as needed.

For example, with https://www.example.com/products/item?color=green#details, the browser would:

• Use "https" to establish a secure connection.

• Connect to "www.example.com" (the web server).

• Navigate to "/products/item" on the server.

• Pass "color=green" to the server.

• Scroll to the "details" section within the retrieved resource.

Key differences between URI and URL

While URIs and URLs are related concepts, it's important to understand their differences.

Hierarchy

• A URI is the overarching term that describes different types of identifiers, such as URLs and URNs.

• URLs are a specific type of URI. All URLs are URIs but not all URIs are URLs.

Purpose

• URIs can be used for both locating and naming/identifying resources across different formats (websites, documents, images, etc.)

• URLs are primarily used for locating resources on the web.

Structure

• URIs can follow different formats and conventions depending on their purpose (e.g., URNs).

• URLs adhere to a specific syntax and structure for locating web resources (scheme, authority, path, query, fragment).

Understanding URIs and URLs Through Examples

To better grasp the concepts of URIs and URLs, let's explore some real-world examples.

URIs that are not URLs (e.g., URNs, mailto)

While all URLs are considered URIs, not all URIs are necessarily URLs. One common type of URI that is not a URL is a URN.

Here are a few examples of URNs:

• urn:isbn:0451450523 (identifies a book by its ISBN number)

• urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6 (a UUID-based URN)

• urn:ietf:rfc:7231 (identifies an IETF RFC document)

As you can see, URNs are designed to uniquely identify resources without specifying their location. They provide persistent, location-independent names for various types of resources.

Here's an example of a "mailto:" scheme:

• mailto:info@example.com (identifies an email address)

The "mailto:" scheme is a type of URI used to identify email addresses, similar to how URNs identify resources without specifying locations.

Common URL Examples

On the other hand, URLs are specifically used to locate and access resources on the internet. Let's explore some common examples:

Web Page Addresses

• https://www.example.com

• http://www.example.org/about

• https://blog.example.net/articles/latest-news

In these URLs, you can see the different components: the scheme (http:// or https://), the domain or authority (www.example.com, www.example.org, blog.example.net), and the path (/about, /articles/latest-news).

File Downloads

• ftp://ftp.example.com/pub/files/document.pdf

• https://www.example.net/downloads/software.zip

These URLs point to files hosted on FTP or web servers, allowing users to download them.

Best Practices for Using URIs and URLs

While URIs and URLs may seem like simple strings of characters, following best practices in their construction and usage can go a long way in ensuring a streamlined and user-friendly experience. Here are some essential guidelines to keep in mind:

Choose the Right Scheme

The scheme component of a URI or URL specifies the protocol or method for accessing the resource. Choose the appropriate scheme based on the resource to ensure clients (e.g., web browsers, email clients) can interpret and handle the URI or URL correctly.

For example:

• Use "https://" for all webpages, even if they don't contain sensitive information or require secure communication.

• Use "ftp://" for accessing and transferring files over the File Transfer Protocol (FTP).

• "mailto:" is the scheme for identifying email addresses as resources.

Create Meaningful and Readable URIs/URLs

Well-structured and descriptive URIs/URLs not only help identify/organize resources but also enhance search engine optimization and contribute to a better user experience.

Consider the following best practices:

• Use clear and concise path segments that accurately reflect the resource hierarchy or structure.

• Incorporate relevant keywords or terms that provide context about the resource.

• Avoid unnecessary or redundant path segments that add complexity without providing meaningful information.

• Separate words in path segments using hyphens (-) or underscores (_) for improved readability.

Handling Special Characters and Encoding

URIs and URLs can contain various special characters, such as spaces, non-ASCII characters, or characters with specific meanings (e.g., /, ?, #).

To ensure proper handling and interpretation, it's important to follow encoding guidelines:

• Replace spaces with "%20" or use the appropriate percent-encoding for non-ASCII characters.

• Encode characters with special meanings (e.g., ?, #, /) using percent-encoding when necessary.

• Avoid using unnecessary special characters or symbols that may cause confusion or compatibility issues.

What is a URN?

Just like a URL, a URN (Uniform Resource Name) is a subset of the URI.

Unlike a URL, URN is used to identify a resource without specifying its location or how to access it. Think of it as a way to name something in a globally unique manner, much like an ISBN for books. While URLs will provide the location of a resource, URNs are designed to provide a consistent way to reference a resource, regardless of where it might be located or how it can be accessed.

For example, the URN for a specific book might look like this: urn:isbn:978-3-16-148410-0 – this identifier will always refer to that particular book, regardless of where you find it or how you access it.

Common Misconceptions about URIs and URLs

Now that we've dispelled the main misconception of URIs and URLs being interchangeable, let's look at a few more common mix-ups often associated with these resource identifiers.

Misconception: URIs and URLs only refer to web addresses

• URIs and URLs can refer to resources beyond just web pages. For example, a URL can point to a file on an FTP server, an email address, or even a database query. Similarly, URIs can identify any kind of resource, including those that are not accessible over the internet.

Misconception: URLs must include "www"

• The "www" prefix is not a mandatory part of a URL. It is simply a subdomain conventionally used by many websites. URLs can function perfectly well without it (e.g., http://example.com vs. http://www.example.com).

Misconception: URLs must include "http://" or "https://"

• While "http://" and "https://" are common schemes for web pages, URLs can use various schemes such as "ftp://", "mailto:", "file://", and more, depending on the protocol needed to access the resource.

Misconception: Case sensitivity in URLs

• URLs are generally case-sensitive, except for the scheme and domain parts. For instance, http://example.com/Page and http://example.com/page may point to different resources. However, the domain part (e.g., "example.com") is case-insensitive.

Misconception: Changing a URL doesn't affect SEO or usability

• Changing URLs can impact both SEO and usability. Search engines index specific URLs, so changing them without proper redirects can lead to broken links and loss of page ranking.

Misconception: URLs cannot contain spaces

• URLs can encode spaces using a special character, typically %20. Spaces are not valid in URLs directly, but they can be represented in an encoded format.

Misconception: URIs and URLs can only identify static content

• URIs and URLs can also identify dynamic content. For example, a URL with query parameters (e.g., http://example.com/search?q=hosting) often points to dynamic content generated by a server-side application based on the input parameters.

Misconception: Only certain characters are allowed in URLs

• While URLs are limited to a specific set of characters, other characters can be included using percent-encoding (e.g., %20 for a space).

Misconception: URL length doesn't matter

• Browsers have practical limits on URL character length, upwards of 80,000+, and while there are not direct consequences from an SEO perspective overly long URLs can be difficult for users to manage or share.