What is a URI?

URI, short for Uniform Resource Identifier, is a character string used to identify a resource, regardless of whether it’s accessible over the internet. It can be a digital document, a printed book, or an XML namespace that defines a particular vocabulary.

A URI can be a locator, name, or both.

For instance, https://www.who.int/about/brochure_en.pdf?ua=1 is the URI for a PDF file from the World Health Organization (WHO). The resource is identified as /about/brochure_en.pdf?ua=1, and it is hosted on who.int.

Meanwhile, urn:isbn:9780877796367 is the URI for the Merriam-Webster Dictionary. It is merely a name to identify the book, without means of accessing the resource.

Syntax of URIs

The syntax of each scheme depends on its type, the part that always begins a URI. 

However, some URIs share several common features and form a generic URI syntax:

• Scheme – a mandatory element that gives information about the protocol being used. The most common scheme is the Hypertext Transfer Protocol Secure (HTTPS).
• Authority – identifies the domains, typically preceded by double slashes (//).
• Path – shows the path to the resource. In the example above, the path is /about/brochure_en.pdf.
• Query – an optional element that represents the request, preceded by a question mark (?).
• Fragment – refers to a specific part of the resource, such as a heading. Typically preceded by a hashtag (#), this element is also optional.

An example of a URI typically looks like this: 

scheme :// authority / path ? query # fragment

Differences between a URI, a URL, and a URN

There are two types of URI: a Uniform Resource Locator (URL) and Uniform Resource Name (URN). As such, URIs can be either URLs or URNs.

URI vs URL

A URL is a type of URI that tells you how to access the resource. It includes the protocol, authority, and the resource path. URLs are commonly called hyperlinks or links in everyday language.

Let’s say you want to share an article or video with a friend. To do so, you need to copy the full URL for your friend to be able to see the exact article or video. 

Each URL is unique to a specific file on the web, and is therefore an important part of navigating the internet.

URLs have their own specific syntax:

• Protocol – this informs web servers on how to access a resource. Common protocols include https://, mailto: to draft and send emails, and ftp:// to transfer files.
• Second-level domain (SLD) – the name of your website, like Zyro.
• Top-level domain (TLD) – this informs the DNS resolver where to look for the resource. .com and .org are common TLDs.
• Subdirectory – defines where the specific file or resource is on the website.

In relation to the generic URI syntax, SLD and TLD form the authority of the resource, while a subdirectory is the resource path.

An example of a URL looks like this:

http://www.zyro.com/blog

URI vs. URN

A URN is another type of URI that identifies a resource by a specific name. It’s commonly used to locate books using a string of numbers called ISBN and ISSN. Online journals also use a URN called DOI, so users can easily find them on the internet.

It’s easy to tell the difference between a URL and a URN. Unlike URLs, URNs do not have a protocol or specific locator like https:// or ftp://. Instead, it simply states the name of the file.

Examples of URLs and URNs

Below is a list of URLs to further illustrate the differences between the types of URI. 

The mandatory component of a URL is the protocol, followed by a colon (:) and double slashes (//). It tells the browser what mechanism is needed to retrieve a source.

• URL using the HTTPS protocol – https://www.zyro.com/
• URL using the mailto protocol – mailto://example@zyro.com
• URL using the FTP protocol – ftp://subdomain.example.com/resource/file.txt

Below is a list of URNs. As you can see, URNs do not specify the location of a resource on the internet. URNs also typically start with urn, which is not a protocol, and therefore not a URL.

• URN to specify an online book – urn:isbn:1234567 
• URN to specify an IETF document  – urn:ietf:rfc:1234

URN to specify a telephone number – urn:tel:+1234567