While scouring the internet, I came across a page about URI and after spending several hours reading, it dawned on me that I knew very little about this topic.



I used to believe that either a URL or URN was considered a URI and that my comprehension was limited to this assumption. However, surprisingly, there is still a lot more to learn about URI. Let's take a closer look.

URI (Uniform Resource Identifier) stands for unified resource identifier and is used to identify any resource; physical or abstract. It serves as an "interface" expressed in OOP where its subtypes provide us with intriguing information.

URN (Uniform Resource Name) is known for being the uniform name of the resource, which can enable you to access the unique resource by name alone. Its structure consists of three elements: "urn:", namespace identifier (NID), and namespace-specific string (NSS).

URL (Uniform Resource Locator), the most commonly used subtype, specifies where to find a resource.

PURL (Persistent Uniform Resource Locator) is similar to URL, but it functions as a constant resource identifier suitable for redirecting resources that have been relocated, deleted, or renamed. Its primary goal is to create a PURL database that maps active URLs to the corresponding PURLs.

IRI (Internationalized Resource Identifier) is a worldwide resource identifier that allows anyone to write the resource's address in any language. IRI is used as a workaround due to the limitations of URL.

XRI (Extensible Resource Identifier) is compatible with IRI and URI. This protocol created by OASIS sets global goals that cannot be contained within a single post. It consists of two layers of identifiers: I-Number and I-Name, allowing better cross-referencing among logical resources.

It's worth noting that the URI doesn't provide any clear delimiters in the query-string. The current practice of using "equal" and "ampersand" as separators is merely due to their popularity. In the past, some web servers employed a semicolon separator instead.

A URI represents the name and address of a resource on the network, and is commonly divided into URL and URN, which both serve as components of URI. URL specifies the address of a resource on the web, indicating its location and how it can be accessed, whereas URN denotes the name of a resource on the web, defining only the name of a specific object that could exist in multiple places.
Hence, it can be inferred that URI can be equal to URL or to URN, or even to URL + URN.

Let's dive a bit deeper into the differences between URI, URL, and URN.

URI (Uniform Resource Identifier) is the overarching term that encompasses both URLs and URNs. It is a string of characters used to identify and locate resources on the internet or other networked systems. The key distinction of a URI is that it serves as a unique identifier for a resource, regardless of whether it specifies its location (URL) or just its name (URN).

URL (Uniform Resource Locator) is a subtype of URI that provides the specific location of a resource on the internet. It includes both the protocol used to access the resource (e.g., HTTP, FTP) and the domain name or IP address where the resource is hosted. In simpler terms, a URL acts as an address by which you can access a particular resource online.

URN (Uniform Resource Name), on the other hand, is another subtype of URI that is designed to provide a persistent and unique name for a resource without specifying its location. Unlike URLs, which can change if the resource is moved or taken down, URNs remain constant. The idea behind URNs is to provide a consistent way to refer to a resource, even if its availability or location changes over time.

In summary, a URI is a generic term for a string of characters that identifies a resource. URLs are a specific type of URI that includes both the location and the means to access a resource. URNs, on the other hand, are a type of URI that provides a persistent name for a resource without specifying where it can be accessed.