science

Uniform Resource Identifier

Uniform Resource Identifier is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Uniform Resource Identifier rather than just read about it. In short: A Uniform Resource Identifier (URI), formerly Universal Resource Identifier, is a unique sequence of characters that identifies an abstract or physical resource, such as resources on a webpage, email address, phone number, books, real-world objects such as people and places, and concepts. In particular, the resource need not be retrievable via the Internet, or any computer network.

Key takeaways

Uniform Resource Identifier belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Uniform Resource Identifier to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Uniform Resource Identifier from memory before moving on to harder problems.

Reference excerpt

A Uniform Resource Identifier (URI), formerly Universal Resource Identifier, is a unique sequence of characters that identifies an abstract or physical resource, such as resources on a webpage, email address, phone number, books, real-world objects such as people and places, and concepts. In particular, the resource need not be retrievable via the Internet, or any computer network. URIs that provide a means of locating and retrieving information resources on a network (like the Internet, an Intranet or a computer file system) are called Uniform Resource Locators (URLs). URLs are therefore a subset of URIs. Other URIs provide only a unique name, without a means of locating or retrieving the resource or information about it; these are Uniform Resource Names (URNs). The web technologies that use URIs are not limited to web browsers.

History

Conception URIs and URLs have a shared history. In 1990, Tim Berners-Lee's proposals for hypertext implicitly introduced the idea of a URL as a short string representing a resource that is the target of a hyperlink. At the time, people referred to it as a "hypertext name" or "document name". Over the next three and a half years, as the World Wide Web's core technologies of HTML, HTTP, and web browsers developed, a need to distinguish a string that provided an address for a resource from a string that merely named a resource emerged. Although not yet formally defined, the term Uniform Resource Locator came to represent the former, and the more contentious Uniform Resource Name came to represent the latter. In July 1992 Berners-Lee's report on the Internet Engineering Task Force (IETF) "UDI (Universal Document Identifiers) BOF" mentions URLs (as Uniform Resource Locators), URNs (originally, as Unique Resource Numbers), and the need to charter a new working group. In November 1992 the IETF "URI Working Group" met for the first time. During the debate over defining URLs and URNs, it became evident that the concepts embodied by the two terms were merely aspects of the fundamental, overarching, notion of resource identification. In June 1994, the IETF published RFC 1630, Berners-Lee's first Request for Comments that acknowledged the existence of URLs and URNs. Most importantly, it defined a formal syntax for Universal Resource Identifiers (i.e. URL-like strings whose precise syntaxes and semantics depended on their schemes). It also attempted to summarize the syntaxes of URL schemes in use at the time. It acknowledged—but did not standardize—the existence of relative URLs and fragment identifiers.

Refinement In December 1994, RFC 1738 formally defined relative and absolute URLs, refined the general URL syntax, defined how to resolve relative URLs to absolute form, and better enumerated the URL schemes then in use. The agreed definition and syntax of URNs had to wait until the publication of IETF RFC 2141 in May 1997. The publication of IETF RFC 2396 in August 1998 saw the URI syntax become a separate specification and most of the parts of RFCs 1630 and 1738 relating to URIs and URLs in general were revised and expanded by the IETF. The new RFC changed the meaning of U in URI from "Universal" to "Uniform." In December 1999, RFC 2732 provided a minor update to RFC 2396, allowing URIs to accommodate IPv6 addresses. A number of shortcomings discovered in the two specifications led to a community effort, coordinated by RFC 2396 co-author Roy Fielding, that culminated in the publication of IETF RFC 3986 in January 2005. While obsoleting the prior standard, it did not render the details of existing URL schemes obsolete; RFC 1738 continues to govern such schemes except where otherwise superseded. IETF RFC 2616 for example, refines the http scheme. Simultaneously, the IETF published the content of RFC 3986 as the full standard STD 66, reflecting the establishment of the URI generic syntax as an official Internet protocol. In 2001, the World Wide Web Consortium's (W3C) Technical Architecture Group (TAG) published a guide to best practices and canonical URIs for publishing multiple versions of a given resource. For example, content might differ by language or by size to adjust for capacity or settings of the device used to access that content. In August 2002, IETF RFC 3305 pointed out that the term "URL" had, despite widespread public use, faded into near obsolescence, and serves only as a reminder that some URIs act as addresses by having schemes implying network accessibility, regardless of any such actual use. As URI-based standards such as Resource Description Framework make evident, resource identification need not suggest the retrieval of resource representations over the Internet, nor need they imply network-based resources at all. The Semantic Web uses the HTTP URI scheme to identify both documents and concepts for practical uses, a distinction which has caused confusion as to how to distinguish the two. The TAG published an e-mail in 2005 with a solution of the problem, which became known as the httpRange-14 resolution. The W3C subsequently published an Interest Group Note titled "Cool URIs for the Semantic Web", which explained the use of content negotiation and the HTTP 303 response code for redirections in more detail.

Design

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Uniform Resource Identifier

Start with the simplest possible case. Write down what Uniform Resource Identifier claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Uniform Resource Identifier before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Uniform Resource Identifier ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Uniform Resource Identifier

In research: Uniform Resource Identifier appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Uniform Resource Identifier in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Uniform Resource Identifier is common in secondary-school and first-year university syllabi. It links to neighbouring topics Application layer protocols, Internet Standards, Internet protocols, so understanding it makes those chapters shorter.
In everyday life: Look for Uniform Resource Identifier outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Uniform Resource Identifier in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Uniform Resource Identifier means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Uniform Resource Identifier out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Uniform Resource Identifier in simple terms?

Why does Uniform Resource Identifier matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Uniform Resource Identifier?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Uniform Resource Identifier.