Understanding the Differences and Relationships: URI vs URL

The subtle differences between URI (Uniform Resource Identifiers) and URL (Uniform Resource Locator) often cause confusion. This article aims to explain these concepts and the key aspects that differentiate them.

Overview Comparison Table

The main difference between a URL (Uniform Resource Locator) and a URI (Uniform Resource Identifier) is that a URL locates a resource on the internet and describes the method for retrieving it, while a URI identifies a resource either by name, location, or both, but does not necessarily provide a means to access it.

What is URI?

A Uniform Resource Identifier (URI) is a sequence of characters that identifies an abstract or physical resource. It serves as a single identifier that would allow the resource to be located. A URI can be a name, a locator, or both for an online resource.

URI is a broad term including Uniform Resource Names (URNs) and URLs. While every URL is a URI, not every URI is a URL as it doesn't necessarily provide the location of the resource. The main purpose of a URI is to provide a simple and extendable way for identifying a resource.

Examples of URI

URI’s formal syntax consists of a sequence of components separated by specific delimiters. The scheme followed by a colon (':'), an authority component, path, query and a fragment.

Examples of URI:


1. Absolute URI: http://www.google.com/search?q=example
2. Relative URI: /images/example.jpg
3. urn:isbn:0451450523

In the first example, you see an absolute URI, which provides the entire address of a web page on the internet.

The second is a relative URI, typically used within the same website.

The third example is a URN (Uniform Resource Name), a type of URI that uses the urn scheme, and doesn't imply availability of the identified resource.

What is URL?

A URL (Uniform Resource Locator) is a specific type of URI that identifies the network location of a resource. It elaborates on how the resource can be retrieved. A URL represents the network location of a web resource, the protocol used to access it, and potentially, the port number and representation metadata.

URLs are the most common form of URI that we encounter on a day-to-day basis. Think of a URL as a street address for a particular website. It's what we type into our web browser's address bar to reach a specific website.

Examples of URL with Code:

URLs have a specific syntax that includes a protocol, domain name, path, and optional parameters. An example of the typical structure can be protocol://domain/path?query.

Examples of URLs:


1. https://www.google.com: This is a simple URL with a secured HTTP (https) protocol and the domain name "google.com".
2. http://localhost:8080/test.html: This URL points to a file named `test.html` on localhost, running on port 8080.
3. https://www.example.com/profile?user=john: This URL points to the profile page of the user "john" in the "example.com" website.

In the examples above, the first URL indicates a secure website, the second indicates a local development environment URL, and the third includes a specific path and query string.

When to Use URI

Understanding when to use a URI versus a URL is just as important as understanding what they mean.

Scenarios Demanding the Use of URI

A URI is your go-to choice when you need to provide a broad, location-independent identifier for a resource. For instance, when designing data systems where each item needs a unique identifier, a URI can be handy. They are also essential when you want to establish a standard way to refer to resources that may not be immediately accessible.

Examples of When to Use URI

Take a look at the following examples.

1. URIs in XML Namespace Definitions:
   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">


2. Say you are developing a library management system where books are referenced by their ISBN, using URIs could be beneficial:
   urn:isbn:1234567890


3. URI in API identifier:
   https://api.example.com/users/{user-id}

In the first example, URI is used in XML namespace definitions, providing each namespace with a unique identifier that does not need to be a retrievable resource.

The second example illustrates URI's usage in a library management system where each book is represented by its unique ISBN number.

And the last example indicates a URI used to identify specific user resources in an API, irrespective of how or where that resource is stored.

When to Use URL

Knowing when to specifically use a URL can streamline your web development and browsing experience.

Instances Where URLs are More Appropriate

As URLs are a subset of URIs that can locate a resource, they are preferable when you need to provide a path for a web browser or other client to retrieve a specific resource from a server. URLs are commonly used for web pages (HTTP, HTTPS), file transfer (FTP), email (mailto), database access (JDBC), and many other applications.

In addition, anytime you want to directly link to a specific online resource — like a website's page, a particular image, or a downloadable file — a URL is what you'll use.

Examples of When to Use URL

Here are a few quick examples below.

1. Image on a CDNs (Content Delivery Networks):
   https://cdn.example.com/images/logo.png


2. File Download:
   https://www.example.com/downloads/application.exe


3. Web page URL in an email or message, hyperlink, or anchor text:
   <a href="https://www.example.com/contact">Contact Us</a>

In the first example, a URL is used to locate and load an image from a CDN. In the second, a URL is used to download a specific file. In the final example, a URL is used in an HTML anchor tag to create a clickable link to a webpage.

How Are URIs and URLs Associated?

The relationship between URIs, URLs, and URNs can be a bit tricky to wrap your head around.

Can a URI be Both a URL and a URN?

Yes, a URI can be both a URL and a URN simultaneously because both are kinds of URI. However, this is relatively uncommon. Think of it this way: an identifier could tell you both where something is (URL) and what its name is (URN).

Connecting URIs, URLs, and URNs

The best way to understand the relationship between URIs, URLs, and URNs is to think of it in terms of rectangles and squares. Conceptually, in the way that a square is a type of rectangle but a rectangle isn't necessarily a square, a URL is a type of URI but a URI isn't necessarily a URL.

Similarly, a URN is also a type of URI, but again, not all URIs are URNs.

IRI, URI, URL, URN and their Differences

Another term you might hear in relation to URLs and URIs is IRI or Internationalized Resource Identifier. An IRI is a version of a URI that includes Unicode characters. This allows IRIs to include scripts such as Arabic, Chinese, Cyrillic, Devanagari, and others that aren't represented in the Latin alphabet.

Here's a brief rundown:

• URIs (Uniform Resource Identifiers) are a broad category that encompasses URLs, URNs, and IRIs.
• URLs (Uniform Resource Locators) tell you where you can find a particular resource.
• URNs (Uniform Resource Names) give a name to a particular resource but don't tell you where you can find it.
• IRIs (Internationalized Resource Identifier) are a version of URI that includes more potential characters.

Understanding the crucial differences and associations among URIs, URLs, URNs, and IRIs is integral for effective web development and management.

Key Takeaways

The world of internet terminologies can often be confusing, and URLs and URIs often top the list of most confusing terms. However, understanding their differences and relationships can help you communicate more effectively and better understand how the internet works.

Main Differences Between URL and URI

The main difference between URL and URI is that a URL is a locator of a resource on the internet, while a URI is an identifier of a resource. Basically, if a string locates a resource, it's a URL. And if a string identifies a resource, it's a URI.

Here are the basic differences in a nutshell:

• A URI can be a name, a locator, or both for a web resource.
• A URL is a type of URI that describes where a web resource is available and the mechanism for retrieving it.
• All URLs can be URIs, but not all URIs are URLs.

In conclusion, the concepts of URL and URI, though often used interchangeably, serve unique roles in the world of networked resources. Once you understand this, the rest of web development becomes a lot simpler.

FAQs

Several questions often arise when discussing the topic of URIs and URLs. Here are some frequently asked questions to clarify your doubts.

Is a URL also a URI?

Yes, every URL is a URI. The difference lies in the fact that a URI represents a broader class of resource identifiers, encompassing both URLs (which locate resources on the internet) and URNs (which provide unique names for resources without necessarily specifying how to locate them.)

Do Any Browsers Actually Know How to Fetch Documents by URN?

Currently, most common browsers (like Chrome or Firefox) do not have built-in support for resolving URNs into retrievable resources. While the URN scheme has been standardized and is used in many contexts, it is not typically used to locate resources on the public internet. If you enter a URN into your web browser's address bar, it's likely not going to know what to do, and will probably just attempt a search for the URN as a string.

But Doesn't the W3C Now Say That URLs and URIs Are the Same Thing?

There has been some overlap in the terminology used by the World Wide Web Consortium (W3C). They have sometimes used the term "URL" to include things that under the formal definitions are URIs not URLs. However, the strict definitions have not changed: URLs are a subset of URIs, and not all URIs are URLs. Whenever there's ambiguity or confusion, it's helpful to return to these formal definitions.