What is the difference between a URI, a URL, and a URN?

Question

What is the difference between a URL, a URI, and a URN?

Hodgkin

URL is more specific than URI. – Backwash

Tor the webmasters take on this question: What is difference between URI and URL – Thilda

I've always understood a URL to be what a user would use to navigate to a location (whether manually typed or copied/pasted) whereas a URI is the server-side interpretation of the original URL. But apparently, there's a lot more to it, as described in the numerous answers. – Leolaleoline

Mini Venn diagram: ( URIs ( URLs ) ) – Coverley

There still seems to be a lot of confusion about URI vs URL, even by those who attempted to answer the question. It would benefit everyone to see practical examples of URLs that are not URIs, examples of URIs that are not URLs, and examples that are URLs and URIs. – Demogorgon

Kathy: "Is that your dog?" Bob: "It would be more correct to call him a canine." Kathy: "No, he's a dog. You, sir, are a pedant." – Glister

@hippietrail, thanks for bi-directional links between the two, much prefer the accepted answer on webmasters stack exchange – Sublimate

Well, URI and URL look the same in some fonts. – Hedy

What is the difference between Vegetable, Tomatoes in the Fridge and Tomatoes? – Rieger

@Rieger : tomatoes are a fruit – Robbi

Answer 1

From RFC 3986:

A URI can be further classified as a locator, a name, or both. The term "Uniform Resource Locator" (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network "location"). The term "Uniform Resource Name" (URN) has been used historically to refer to both URIs under the "urn" scheme [RFC2141], which are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable, and to any other URI with the properties of a name.

So all URLs are URIs, and all URNs are URIs - but URNs and URLs are different, so you can't say that all URIs are URLs.

If you haven't already read Roger Pate's answer, I'd advise doing so as well.

Vaclav

Only URIs with the urn: scheme are URNs. A URI could be a classic URL, a URN, or just a URI that doesn't start with "urn:" and doesn't refer to a location of a resource. – Lepanto

One example are InfoPath URNs, which are important i.e. in Sharepoint Workflows: urn:schemas-microsoft-com:office:infopath:blogsample:-myXSD-2004-05-19T20-48-18 – Hinge

Not "all URLs are URIs". It depends on the interpretation of the RFC. For example in Java the URI parser does not like [ or ] and that's because the spec says "should not" and not "shall not". – Oink

@AdamGent: RFC 3986 1.1.3: "A URI can be further classified as a locator, a name, or both." So, if URL is a special kind of URI, that means that every URL is a URI. Doesn't it? – Exultation

It depends on your semantics of "special". URLs are not a syntactical subset of URI just like C is not truly a syntacticaly a subset of C++ (IIRC ... That might have changed). – Oink

@AdamGent: That sounds like a Java implementation quirk, and not normative. The java.net.URI doc itself says "every URL is a URI, abstractly speaking, but not every URI is a URL". And java.net.URL does weird stuff like checking equality of URLs by resolving host names to IP addresses (which seems at odds with RFC 3986 sec 6 in the first place, and breaks w virtual hosts). I think this just means the Java Standard Library has some inconsistent class behavior. – Aut

I think I figured out the Java [ and ] bracket thing. RFC 3986 sec 3.2.2 says "[IPv6 host addresses] is the only place where square bracket characters are allowed in the URI syntax." If you use them in the right place, java.net.URI accepts them. new java.net.URI("foo://[1080:0:0:0:8:800:200C:417A]/a/b") succeeds for me in Java 1.7. "foo://example.com/a[b]" errors. This sounds in line with the RFC. The form new URI("http", "example.com", "/a/b[c]!$&'()*+", null, null) will %-encode the []. That java.net.URL accepts them elsewhere sounds like it's doing less validation. – Aut

@AndrewJanke: Thanks - not entirely sure where that leaves my answer, admittedly :) – Vaclav

@AndrewJanke Actually its the opposite. The URI parser in Java is stricter (as in it follows the should not) and yes other languages like Python have this problem : urlparse.urlparse('http://blah.com/a[b') and urlparse.parse_qsl('a=[',True,True) should fail but does not (notice how I even passed the strict option on that last one). I blogged about this here which has some more relevant links: adamgent.com/post/25161273526/urls-are-not-a-subset-of-uris . You should also know all of the servlet containers (I have used) will happily pass you URLs with [ and ] in the path. – Oink

@AdamGent Maybe I need to do some more reading on this. Where is the "should not" you're referring to? In RFCs back to RFC 1630, I only see them saying that [ and ] "must" be escaped or "may not" appear. – Aut

@AndrewJanke I included the link in my blog post (you might have missed it) tools.ietf.org/html/rfc2396#section-2.4.3 (the continued page 10). Yes that is RFC 2396 but most things probably follow that older spec instead of RFC 3986 which I haven't looked at extensively. – Oink

@AndrewJanke The characters like [, ] are called unwise hence the should not. I can't find where it says should not for unwise (maybe an older spec) but its definitely inferred. – Oink

@AdamGent Yes, I read it from your blog post. The word "should" doesn't appear in section 2.4.3. The normative language is in the last sentence, and it's a "must": "Data corresponding to excluded characters must be escaped in order to be properly represented within a URI." Maybe it's a different section? – Aut

@AndrewJanke I'm not really sure. My goal just like my blog post is not really to be pedantic but informative for others to avoid the pitfalls I have had. In the new spec for URI, URL might be a subset of URI but in the wild that is certainly not the case. – Oink

@JonSkeet Maybe just need to differentiate between standards vs implementations? E.g. "Formally, according to RFCs, all URLs are URIs. (RFC excerpt.) But existing implementations may not match the spec exactly, possibly for interoperability, and may use URLs that are not valid per the RFCs. And because it's a complicated area, some people and documents might use 'URL' to mean something different from the RFC-specified thing." Sort of like how most email validation routines don't match the RFC definitions. – Aut

let us continue this discussion in chat – Aut

@AndrewJanke @Jon Skeet Here is a SO answer to the "should" on [,]: https://mcmap.net/q/18459/-which-characters-make-a-url-invalid – Oink

Answer 2

URIs identify and URLs locate; however, locators are also identifiers, so every URL is also a URI, but there are URIs which are not URLs.

Examples

• Roger Pate

This is my name, which is an identifier. It is like a URI, but cannot be a URL, as it tells you nothing about my location or how to contact me. In this case it also happens to identify at least 5 other people in the USA alone.

• 4914 West Bay Street, Nassau, Bahamas

This is a locator, which is an identifier for that physical location. It is like both a URL and URI (since all URLs are URIs), and also identifies me indirectly as "resident of..". In this case it uniquely identifies me, but that would change if I get a roommate.

I say "like" because these examples do not follow the required syntax.

Popular confusion

From Wikipedia:

In computing, a Uniform Resource Locator (URL) is a subset of the Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it. In popular usage and in many technical documents and verbal discussions it is often incorrectly used as a synonym for URI, ... [emphasis mine]

Because of this common confusion, many products and documentation incorrectly use one term instead of the other, assign their own distinction, or use them synonymously.

URNs

My name, Roger Pate, could be like a URN (Uniform Resource Name), except those are much more regulated and intended to be unique across both space and time.

Because I currently share this name with other people, it's not globally unique and would not be appropriate as a URN. However, even if no other family used this name, I'm named after my paternal grandfather, so it still wouldn't be unique across time. And even if that wasn't the case, the possibility of naming my descendants after me make this unsuitable as a URN.

URNs are different from URLs in this rigid uniqueness constraint, even though they both share the syntax of URIs.

Raybourne

URNs are different from URLs in this rigid uniqueness constraint Does this mean that URLs don't uniquely identify a location? – Showily

Roger's answer provides good pragmatic advice. For the official answer I go to the W3C who published "URIs, URLs, and URNs: Clarifications and Recommendations" in 2001. In a nutshell, W3C says the contemporary view is that everything is a URI. URL is an informal concept, not a formal concept. And the confusion dates back to a "classical view" which tried to rigidly distinguish between categories of URI (of which URL was one category). – Qintar

..a Uniform Resource Locator (URL).. specifies where an identified resource is available and the mechanism for retrieving it. So in other words, there is no such thing as a "relative" URL? – Diecious

Is "earth128:Edward-de-Leau/6000000000569063853" (the unique me over multiple multiverse) a URN, a URL or a URI? – Grous

@edelwater: I suppose thats a uri as it only identifies you but says nothing of how to get to you, unless you mean earth128 is some medium of inter-planetary travel :) – Written

"Each URI begins with a scheme name" - so your name can't be an example of a URI. Given examples of aren't really examples of URI and URL but an example of "something that is kinda identifier and something that is like a locator". So examples of who knows what really. This is not useful at all. If you start with "URI is this, URL is that, Examples: ..." one would expect you to follow with examples of URI and URL, not "something kinda like that". – Outsider

@Showily URLs don't uniquely identify a location. For example if you and I both enter https://localhost into the browser's address bar, we'll end up in different places. – Matthew

Answer 3

URI -- Uniform Resource Identifier

URIs are a standard for identifying documents using a short string of numbers, letters, and symbols. They are defined by RFC 3986 - Uniform Resource Identifier (URI): Generic Syntax. URLs, URNs, and URCs are all types of URI.

URL -- Uniform Resource Locator

Contains information about how to fetch a resource from its location. For example:

• http://example.com/mypage.html
• ftp://example.com/download.zip
• mailto:[email protected]
• file:///home/user/file.txt
• tel:[phone]
• http://example.com/resource?foo=bar#fragment
• /other/link.html (A relative URL, only useful in the context of another URL)

URLs always start with a protocol (http) and usually contain information such as the network host name (example.com) and often a document path (/foo/mypage.html). URLs may have query parameters and fragment identifiers.

URN -- Uniform Resource Name

Identifies a resource by a unique and persistent name, but doesn't necessarily tell you how to locate it on the internet. It usually starts with the prefix urn: For example:

• urn:isbn:0451450523 to identify a book by its ISBN number.
• urn:uuid:6e8bc430-9c3a-11d9-9669-0800200c9a66 a globally unique identifier
• urn:publishing:book - An XML namespace that identifies the document as a type of book.

URNs can identify ideas and concepts. They are not restricted to identifying documents. When a URN does represent a document, it can be translated into a URL by a "resolver". The document can then be downloaded from the URL.

URC -- Uniform Resource Citation

Points to meta data about a document rather than to the document itself. An example of a URC is one that points to the HTML source code of a page like: view-source:http://example.com/

Data URI

Rather than locating it on the internet, or naming it, data can be placed directly into a URI. An example would be data:,Hello%20World.

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Frequently Asked Questions

I've heard that I shouldn't say URL anymore, why?

The W3 spec for HTML says that the href of an anchor tag can contain a URI, not just a URL. You should be able to put in a URN such as <a href="urn:isbn:0451450523">. Your browser would then resolve that URN to a URL and download the book for you.

Do any browsers actually know how to fetch documents by URN?

Not that I know of, but modern web browser do implement the data URI scheme.

Does the difference between URL and URI have anything to do with whether it is relative or absolute?

No. Both relative and absolute URLs are URLs (and URIs.)

Does the difference between URL and URI have anything to do with whether it has query parameters?

No. Both URLs with and without query parameters are URLs (and URIs.)

Does the difference between URL and URI have anything to do with whether it has a fragment identifier?

No. Both URLs with and without fragment identifiers are URLs (and URIs.)

Does the difference between URL and URI have anything to do with what characters are permitted?

No. URLs are defined to be a strict subset of URIs. If a parser allows a character in a URL but not in a URI, there is a bug in the parser. The specs go into great detail about which characters are allowed in which parts of URLs and URIs. Some characters may be allowed only in some parts of the URL, but characters alone are not a difference between URLs and URIs.

But doesn't the W3C now say that URLs and URIs are the same thing?

Yes. The W3C realized that there is a ton of confusion about this. They issued a URI clarification document that says that it is now OK to use the terms URL and URI interchangeably (to mean URI). It is no longer useful to strictly segment URIs into different types such as URL, URN, and URC.

Can a URI be both a URL and a URN?

The definition of URN is now looser than what I stated above. The latest RFC on URIs says that any URI can now be a URN (regardless of whether it starts with urn:) as long as it has "the properties of a name." That is: It is globally unique and persistent even when the resource ceases to exist or becomes unavailable. An example: The URIs used in HTML doctypes such as http://www.w3.org/TR/html4/strict.dtd. That URI would continue to name the HTML4 transitional doctype even if the page on the w3.org website were deleted.

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Visby

is "C:\myfile" an URI,URL or URN ? or none of them. – Barfuss

A file path is not a URL or URI unless you put the file:// prefix on it. Although browsers do generally handle non-URL formatted file paths. Mozilla publishes their test cases for file URLs. – Visby

See section 1.1 of the RFC -- "Uniformity provides several benefits. It allows different types of resource identifiers to be used in the same context, even when the mechanisms used to access those resources may differ. It allows uniform semantic interpretation of common syntactic conventions across different types of resource identifiers..." – Visby

This answer is a lot more easy to understand. I can see the clearly pictures of real example of the URLs & URN. And for anyone to read more about this... danielmiessler.com/study/url-uri – Metal

I do not see where the clarification document says that it is now OK to use the terms URL and URI interchangeably. It says that this is how it appears to people, which seems to suggest that they are not. It also says that people "are not convinced that URI and URL mean the same thing", which seems to suggest that they are. However, there is no explicit statement on this. Ironically, the document adds to my confusion rather than clarifying it. – Tautomer

Answer 4

In summary: a URI identifies, a URL identifies and locates.

Consider a specific edition of Shakespeare's play Romeo and Juliet, of which you have a digital copy on your home network.

You could identify the text as urn:isbn:0-486-27557-4.
That would be a URI, but more specifically a URN* because it names the text.

You could also identify the text as file://hostname/sharename/RomeoAndJuliet.pdf.
That would also be a URI, but more specifically a URL because it locates the text.

*Uniform Resource Name

(Note that my example is adapted from Wikipedia)

Sputter

It's helpful to note the actual URN (to see how it compares to a URL): urn:isbn:0-486-27557-4 – Prejudicial

@Michael - It is my understanding that ISBN 0486275574 also names the text and thus qualify as a URN. I choose a format that I believed would be more familiar to readers. – Sputter

So would it make sense to say that the hash (e.g. SHA1) of a file could be a URN for that file? – Asis

@Asis Don't think so, as you could have a copy of one file on the same computer, which would result into the same hash and therefore it's not unique. – Alegre

Answer 5

These are some very well-written but long-winded answers. Here is the difference as far as CodeIgniter is concerned:

URL - http://example.com/some/page.html

URI - /some/page.html

Put simply, URL is the full way to indentify any resource anywhere and can have different protocols like FTP, HTTP, SCP, etc.

URI is a resource on the current domain, so it needs less information to be found.

In every instance that CodeIgniter uses the word URL or URI this is the difference they are talking about, though in the grand-scheme of the web, it is not 100% correct.

Thorndike

This answer may be over-simplified but look at the context of his question. It will be more helpful to him that waffling on about XML namespaces! – Thorndike

This answer is not only wrong but actively misleading. Both examples are URLs. And since every URL is also a URI, this means that both examples are URIs. For the purpose of demonstrating the difference between URIs and URLs, this is totally useless. – Wicked

This is the difference as far as CodeIgniter is concerned. In every instance they use the word URL or URI this is the difference they are talking about. Therefore in the grand-scheme of the web, it is not 100% correct but in the scope of the OP's question (the difference in CodeIgniter), this answer is perfectly correct. – Thorndike

@Phil Sturgeon - agreed, for the purpose of this question, this is how CI distinguishes between URL and URI. – Todd

@Phil Sturgeon - for the purpose of the question your answer is perfectly fine – Babel

This is wrong. @JörgWMittag is mostly on point. URLs are URIs, and they're "fully qualified"; so the "URL" in this answer is both. But /some/page.html is not a URI. It is a "relative-ref", which is a kind of "URI-reference". Combined with a base URI context, it can be resolved to a URI, but is not itself a URI. See Section 4.1 of RFC 3986. CodeIgniter's probably using the terms wrong and that should be called out; the Q (as currently edited) isn't framed as CodeIgniter-specific. – Aut

When I answered the question it was tagged under #codeigniter. so it was answered as such. – Thorndike

It seems, not only CI but also at least PHP's $_SERVER['REQUEST_URI'] distinguishes like this. – Mcclellan

If /some/page.html is not an URI then it is definitely not an URL - it's not complete enough for me to locate the resource. – Complexity

For future people who read these comments and are as confused as I was: This answer wasn't posted for this question. This question never had anything to do with CodeIgniter. There was a duplicate question which specifically mentioned CodeIgniter which was closed and had all of its answers migrated over to this question. This answer was one of those which were moved from the old closed question to this protected question. Even so, I this answer is misleading. I have downvoted it - others should do the same since, in its new home, it is wrong. The author should delete it or the merge be undone. – Ochre

This answer fails to mention URNs, which are included in the question. Also, original question does not presently mention CodeIgniter. Based on some other comments, perhaps the question previously did mention CodeIgniter though. – Lapoint

Answer 6

First of all get your mind out of confusion and take it simple and you will understand.

URI => Uniform Resource Identifier Identifies a complete address of resource i-e location, name or both.

URL => Uniform Resource Locator Identifies location of the resource.

URN => Uniform Resource Name Identifies the name of the resource

Example

We have address https://www.google.com/folder/page.html where,

URI(Uniform Resource Identifier) => https://www.google.com/folder/page.html

URL(Uniform Resource Locator) => https://www.google.com/

URN(Uniform Resource Name) => /folder/page.html

URI => (URL + URN) or URL only or URN only

Bona

This answer omits the critical aspect of URNs that they be unique over time and space. The URN example is wrong. The bottom URI formula is wrong. – Lapoint

Answer 7

Identity = Name with Location

Every URL(Uniform Resource Locator) is a URI(Uniform Resource Identifier), abstractly speaking, but every URI is not a URL. There is another subcategory of URI is URN (Uniform Resource Name), which is a named resource but do not specify how to locate them, like mailto, news, ISBN is URIs. Source

URN:

• URN Format : urn:[namespace identifier]:[namespace specific string]
• urn: and : stand for themselves.
• Examples:
  • urn:uuid:6e8bc430-9c3a-11d9-9669-0800200c9a66
  • urn:ISSN:0167-6423
  • urn:isbn:096139210x
  • Amazon Resource Names (ARNs) is a uniquely identify AWS resources.
    • ARN Format : arn:partition:service:region:account-id:resource

URL:

• URL Format : [scheme]://[Domain][Port]/[path]?[queryString]#[fragmentId]
• :,//,? and # stand for themselves.
• schemes are https,ftp,gopher,mailto,news,telnet,file,man,info,whatis,ldap...
• Examples:
  • http://ip_server/path?query
  • ftp://ip_server/path
  • mailto:email-address
  • news:newsgroup-name
  • telnet://ip_server/
  • file://ip_server/path_segments
  • ldap://hostport/dn?attributes?scope?filter?extensions

Analogy:
To reach a person: Driving(protocol others SMS, email, phone), Address(hostname other phone-number, emailid) and person name(object name with a relative path).

Niobium

Minor quibble: There should be a colon between [domain] and [port]. IE: example.com:1234 – Clonus

I have to wonder: how does one retrieve the resource associated with a mailto URL ? – Tenderize

A precise spatial position is both an URN, an URL and an URI. Your answer is incorrect and obscure. If you need 20 examples to exemplify a definition, your definition is not clear. – Pyrotechnic

Answer 8

A small addition to the answers already posted, here's a Venn's diagram to sum up the theory (from Prateek Joshi's beautiful explanation):

And an example (also from Prateek's website):

Copilot

I believe the second illustration is incorrect. By the specification url.spec.whatwg.org/#url-writing A URL must be written as either a relative URL or an absolute URL, optionally followed by "#" and a fragment. So, #posts fragment identifier could be part of the URL – Atonement

The two illustrations contradict each other. – Kilometer

URN's should be unique, which is not the case for any subset of URLs. Misleading, if not wrong. – Pyrotechnic

This is incorrect. URN just means an URL that is unique over space and time, i.e, an identifier created exactly once, similar to a UUID. – Otter

Answer 9

This is one of the most confusing and possibly irrelevant topics I've encountered as a web professional.

As I understand it, a URI is a description of something, following an accepted format, that can define both or either the unique name (identification) of something or its location.

There are two basic subsets:

• URLs, which define location (especially to a browser trying to look up a webpage) and
• URNs, which define the unique name of something.

I tend to think of URNs as being similar to GUIDs. They are simply a standardized methodology for providing unique names for things. As in the namespace declarative that uses a company's name—it's not like there is a resource sitting on a server somewhere to correspond to that line of text—it simply uniquely identifies something.

I also tend to completely avoid the term URI and discuss things only in terms of URL or URN as appropriate, because it causes so much confusion. The question we should really try answering for people isn't so much the semantics, but how to identify when encountering the terms whether or not there is any practical difference in them that will change the approach to a programming situation. For example, if someone corrects me in conversation and says, "oh, that's not a URL it's a URI" I know they're full of it. If someone says, "we're using a URN to define the resource," I'm more likely to understand we are only naming it uniquely, not locating it on a server.

If I'm way off base, please let me know!

Subdeacon

No, I think you're right. The semantics of URI vs URL vs URL vs URI-ref etc. are useless to most developers, only because it drives pointless (non-productive, insignificant to decision making) debate. If the Google API used redirect_url instead of redirect_uri, would anyone really care? – Tearle

Answer 10

URI => http://en.wikipedia.org/wiki/Uniform_Resource_Identifier

URL's are a subset of URI's (which also contain URNs).

Basically, a URI is a general identifier, where a URL specifies a location and a URN specifies a name.

Mostly

URLs are not a true subset of URI. You can make vaid URL's with characters [ and ] but not a URI. – Oink

Square brackets are not valid in either URIs or URLs. See this question which has many references to the specs: Are square brackets permitted in URLs?. When square brackets appear in either, they should be encoded. – Visby

Answer 11

Another example I like to use when thinking about URIs is the xmlns attribute of an XML document:

<rootElement xmlns:myPrefix="com.mycompany.mynode">
    <myPrefix:aNode>some text</myPrefix:aNode>
</rootElement>

In this case com.mycompany.mynode would be a URI that uniquely identifies the "myPrefix" namespace for all of the elements that use it within my XML document. This is NOT a URL because it is only used to identify, not to locate something per se.

Padauk

Answer 12

They're the same thing. A URI is a generalization of a URL. Originally, URIs were planned to be divided into URLs (addresses) and URNs (names) but then there was little difference between a URL and URI and http URIs were used as namespaces even though they didn't actually locate any resources.

Lepanto

I thought it was the other way around. A URL refers to a concrete object, and a URI can refer to that or a concept or anything else. – Calli

A URL locates a resource and is a kind of URI, which identifies a resource. – Lepanto

It is only true that they are the same thing because the definition of URL has changed over time. URLs used to be a specific type of URI, but because of the confusion that caused, the W3C redefined URL to mean URI. – Visby

Answer 13

Due to difficulties to clearly distinguish between URI and URL, as far as I remember W3C does not make a difference any longer between URI and URL (http://www.w3.org/Addressing/).

Meares

Maybe I missed that part, but I don't see any reference in the provided link to them removing the distinction between URL and URI, only acknowledging the confusion and wanting specs that are incorrectly referencing URL to be updated to reference URI instead. – Chert

Answer 14

URI, URL, URN

As the image above indicates, there are three distinct components at play here. It’s usually best to go to the source when discussing matters like these, so here’s an exerpt from Tim Berners-Lee, et. al. in RFC 3986: Uniform Resource Identifier (URI): Generic Syntax:

A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource.

A URI can be further classified as a locator, a name, or both. The term “Uniform Resource Locator” (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network “location”).

Jaime

Answer 15

URI is kind of the super class of URL's and URN's. Wikipedia has a fine article about them with links to the right set of RFCs.

Gaia

Answer 16

Wikipedia will give all the information you need here. Quoting from http://en.wikipedia.org/wiki/URI:

A URL is a URI that, in addition to identifying a resource, provides means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network "location".

Airlie

Answer 17

URL

A URL is a specialization of URI that defines the network location of a specific resource. Unlike a URN, the URL defines how the resource can be obtained. We use URLs every day in the form of http://example.com etc. But a URL doesn't have to be an HTTP URL, it can be ftp://example.com etc., too.

URI

A URI identifies a resource either by location, or a name, or both. More often than not, most of us use URIs that defines a location to a resource. The fact that a URI can identify a resources by both name and location has lead to a lot of the confusion in my opinion. A URI has two specializations known as URL and URN.

Difference between URL and URI

A URI is an identifier for some resource, but a URL gives you specific information as to obtain that resource. A URI is a URL and as one commenter pointed out, it is now considered incorrect to use URL when describing applications. Generally, if the URL describes both the location and name of a resource, the term to use is URI. Since this is generally the case most of us encounter everyday, URI is the correct term.

Bravar

Answer 18

A URI identifies a resource either by location, or a name, or both. More often than not, most of us use URIs that defines a location to a resource. The fact that a URI can identify a resources by both name and location has lead to a lot of the confusion in my opinion. A URI has two specializations known as URL and URN.

A URL is a specialization of URI that defines the network location of a specific resource. Unlike a URN, the URL defines how the resource can be obtained. We use URLs every day in the form of http://stackoverflow.com, etc. But a URL doesn’t have to be an HTTP URL, it can be ftp://example.com, etc.

Insect

Answer 19

As per RFC 3986, URIs are comprised of the following pieces:

scheme://authority/path?query

The URI describes the protocol for accessing a resource (path) or application (query) on a server (authority).

All the URLs are URIs, and all the URNs are URIs, but all the URIs are not URLs.

Please refer for more details:

Wikipedia

Mousetrap

This doesn't teach me anything that's not covered by the other answers that are at least 6 years old, and which are much more complete and actually try to explain how to distinguish URIs from URLs. – Israelitish

It is important to note that the image is a Venn diagram even though it doesn't look like a typical one. I've seen people try to interpret it as "parts of the URL". This diagram does not say that URIs start with a URL and end with a URN. – Visby

Answer 20

Although the terms URI and URL are strictly defined, many use the terms for other things than they are defined for.

Let’s take Apache for example. If http://example.com/foo is requested from an Apache server, you’ll have the following environment variables set:

• REDIRECT_URL: /foo
• REQUEST_URI: /foo

With mod_rewrite enabled, you will also have these variables:

• REDIRECT_SCRIPT_URL: /foo
• REDIRECT_SCRIPT_URI: http://example.com/foo
• SCRIPT_URL: /foo
• SCRIPT_URI: http://example.com/foo

This might be the reason for some of the confusion.

Kleiman

Answer 21

See this document. Specifically,

a URL is a type of URI that identifies a resource via a representation of its primary access mechanism (e.g., its network "location"), rather than by some other attributes it may have.

It's not an extremely clear term, really.

Fulk

Answer 22

After reading through the posts, I find some very relevant comments. In short, the confusion between the URL and URI definitions is based in part on which definition depends on which and also informal use of the word URI in software development.

By definition URL is a subset of URI [RFC2396]. URI contain URN and URL. Both URI and URL each have their own specific syntax that confers upon them the status of being either URI or URL. URN are for uniquely identifying a resource while URL are for locating a resource. Note that a resource can have more than one URL but only a single URN.[RFC2611]

As web developers and programmers we will almost always be concerned with URL and therefore URI. Now a URL is specifically defined to have all the parts scheme:scheme-specific-part, like for example https://stackoverflow.com/questions. This is a URL and it is also a URI. Now consider a relative link embedded in the page such as ../index.html. This is no longer a URL by definition. It is still what is referred to as a "URI-reference" [RFC2396].

I believe that when the word URI is used to refer to relative paths, "URI-reference" is actually what is being thought of. So informally, software systems use URI to refer to relative pathing and URL for the absolute address. So in this sense, a relative path is no longer a URL but still URI.

Kristopherkristos

Answer 23

URIs came about from the need to identify resources on the Web, and other Internet resources such as electronic mailboxes in a uniform and coherent way. So, one can introduce a new type of widget: URIs to identify widget resources or use tel: URIs to have web links cause telephone calls to be made when invoked.

Some URIs provide information to locate a resource (such as a DNS host name and a path on that machine), while some are used as pure resource names. The URL is reserved for identifiers that are resource locators, including 'http' URLs such as http://stackoverflow.com, which identifies the web page at the given path on the host. Another example is 'mailto' URLs, such as mailto:[email protected], which identifies the mailbox at the given address.

URNs are URIs that are used as pure resource names rather than locators. For example, the URI: mid:[email protected] is a URN that identifies the email message containing it in its 'Message-Id' field. The URI serves to distinguish that message from any other email message. But it does not itself provide the message's address in any store.

Decimate

Answer 24

Here is my simplification:

URN: unique resource name, i.e. "what" (eg urn:issn:1234-5678 ). This is meant to be unique .. as in no two different docs can have the same urn. A bit like "uuid"

URL: "where" to find it ( eg https://google.com/pub?issnid=1234-5678 .. or ftp://somesite.com/doc8.pdf )

URI: can be either a URN or a URL. This fuzzy definition is thanks to RFC 3986 produced by W3C and IETF.

The definition of URI has changed over the years, so it makes sense for most people to be confused. However, you can now take solace in the fact that you can refer to http://somesite.com/something as either a URL or URI ... an you will be right either way (at least fot the time being anyway...)

Hawser

Answer 25

In order to answer this I'll lean on an answer I modified to another question. A good example of a URI is how you identify an Amazon S3 resource. Let's take:

s3://www-example-com/index.html _{[fig. 1]}

which I created as a cached copy of

http://www.example.com/index.html _{[fig. 2]}

in Amazon's S3-US-West-2 datacenter.

Even if StackOverflow would allow me to hyperlink to the s3:// protocol scheme, it wouldn't do you any good in locating the resource. Because it Identifies a Resource, fig. 1 is a valid URI. It is also a valid URN, because Amazon requires that the bucket (their term for the authority portion of the URI) be unique across datacenters. It is helpful in locating it, but it does not indicate the datacenter. Therefore it does not work as a URL.

So, how do URI, URL, and URN differ in this case?

• fig. 1 is a URI
• fig. 1 is a URN
• fig. 2 is a URI
• fig. 2 is a URL
• The URL for fig. 1 is http://www-example-com.s3-website-us-west-2.amazonaws.com/
  • also http://www-example-com.s3.amazonaws.com/index.html
  • but not http://www-example-com.s3.amazonaws.com/ (no datacenter and no filename is too generic for Amazon S3)

NOTE: RFC 3986 defines URIs as scheme://authority/path?query#fragment

Palmirapalmistry

Answer 26

The best (technical) summary imo is this one

IRI, URI, URL, URN and their differences from Jan Martin Keil:

IRI, URI, URL, URN and their differences

Everybody dealing with the Semantic Web repeatedly comes across the terms IRI, URI, URL and URN. Nevertheless, I frequently observe that there is some confusion about their exact meaning. And, of course, others noticed that as well (see e.g. RFC3305 or search on Google). To be honest, I even was confused myself at the outset. But actually the issue is not that complex. Let’s have a look on the definitions of the mentioned terms to see what the differences are:

URI

A Uniform Resource Identifier is a compact sequence of characters that identifies an abstract or physical resource. The set of characters is limited to US-ASCII excluding some reserved characters. Characters outside the set of allowed characters can be represented using Percent-Encoding. A URI can be used as a locator, a name, or both. If a URI is a locator, it describes a resource’s primary access mechanism. If a URI is a name, it identifies a resource by giving it a unique name. The exact specifications of syntax and semantics of a URI depend on the used Scheme that is defined by the characters before the first colon. [RFC3986]

URN

A Uniform Resource Name is a URI in the scheme urn intended to serve as persistent, location-independent, resource identifier. Historically, the term also referred to any URI. [RFC3986] A URN consists of a Namespace Identifier (NID) and a Namespace Specific String (NSS): urn:: The syntax and semantics of the NSS is specific specific for each NID. Beside the registered NIDs, there exist several more NIDs, that did not go through the official registration process. [RFC2141]

URL

A Uniform Resource Locator is a URI that, in addition to identifying a resource, provides a means of locating the resource by describing its primary access mechanism [RFC3986]. As there is no exact definition of URL by means of a set of Schemes, "URL is a useful but informal concept", usually referring to a subset of URIs that do not contain URNs [RFC3305].

IRI

An Internationalized Resource Identifier is defined similarly to a URI, but the character set is extended to the Universal Coded Character Set. Therefore, it can contain any Latin and non Latin characters except the reserved characters. Instead of extending the definition of URI, the term IRI was introduced to allow for a clear distinction and avoid incompatibilities. IRIs are meant to replace URIs in identifying resources in situations where the Universal Coded Character Set is supported. By definition, every URI is an IRI. Furthermore, there is a defined surjective mapping of IRIs to URIs: Every IRI can be mapped to exactly one URI, but different IRIs might map to the same URI. Therefore, the conversion back from a URI to an IRI may not produce the original IRI. [RFC3987]

Summarizing we can say:

IRI is a superset of URI (IRI ⊃ URI)
URI is a superset of URL (URI ⊃ URL)
URI is a superset of URN (URI ⊃ URN)
URL and URN are disjoint (URL ∩ URN = ∅)

Conclusions for Semantic Web Issues

RDF explicitly allows to use IRIs to name entities [RFC3987]. This means that we can use almost every character in entity names. On the other hand, we often have to deal with early state software. Thus, it is not unlikely to run into problems using non ASCII characters. Therefore, I suggest to avoid non URI names for entities and recommend to use http URIs [LINKED-DATA]. To put it briefly: only use URLs to name your entities. Of course, we can refer to existing entities named by a URN. However, we should avoid to newly create this kind of identifiers.

Xenia

This is very nice, but a picture of an example would have been far more useful. I.e. Show me these different parts of: httsp://www.google.com/lala/example?something=1. – Applaud

Answer 27

I was wondering about the same thing and I've found this: http://docs.kohanaphp.com/helpers/url.

You can see a clear example using the url::current() method. If you have this URL: http://example.com/kohana/index.php/welcome/home.html?query=string then using url:current() gives you the URI which, according to the documentation, is: welcome/home

Marshall

This answer is wrong. A URI is not a portion of the URL. Rather URLs are a type of URI. Furthermore, the link in this answer is broken (and I can not find a suitable replacement.) – Visby

Answer 28

Easy to explain:

Lets assume the following

URI is your Name

URL is your address with your name in-order to communicate with you.

• my name is Loyola

  Loyola is URI

• my address is TN, Chennai 600001.

TN, Chennai 600 001, Loyola is URL

Hope you understand,

Now lets see a precise example

http://www.google.com/fistpage.html

in the above you can communicate with a page called firstpage.html (URI) using following http://www.google.com/fistpage.html(URL).

Hence URI is subset of URL but not vice-versa.

Animism

This answer is misleading. Quote from Wikipedia "A Uniform Resource Name (URN) functions like a person's name, while a Uniform Resource Locator (URL) resembles that person's street address. In other words: the URN defines an item's identity, while the URL provides a method for finding it." Also both URNs and URLs are URIs. – Cacodemon

Answer 29

I found:

---

A uniform resource identifier(URI) represents something of a big picture. You can split URIs/ URIs can be classified as locators (uniform resource locators- URL), or as names (uniform resource name-URN), or either both. So basically, a URN functions like a person's name and the URL depicts that person's address. So long story short, a URN defines an item's identity, while the URL provides defines the method for finding it, finally encapsulating these two concepts is the URI

Accoutre

Do you recall the source for where you found this? Was it Wikipedia or somewhere else? It is okay if it is Wikipedia! I am not trying to be difficult about attribution but rather, I am asking because your answer is different than this one, and I think that your answer is correct. – Seaweed

Answer 30

The answer is ambiguous. In Java it is frequently used in this way:

An Uniform Resource Locator (URL) is the term used to identify an Internet resource including the scheme( http, https, ftp, news, etc.). For instance What is the difference between a URI, a URL and a URN?

An Uniform Resource Identifier (URI) is used to identify a single document in the Web Server: For instance /questions/176264/whats-the-difference-between-a-uri-and-a-url

In Java servlets, the URI frequently refers to the document without the web application context.

Quaternity

This is the difference between an absolute and a relative URL. It doesn't explain the relation of URI vs. URL and URN. – Witcher

Actually, these are accurate examples. On the Web, this is very often the difference. – Proofread

Answer 31

Everybody seems to have their own answer to this question. Me trying to give proper naming to arguments used in API sessions... ty so much for the confusion all.

My source of truth is gonna be the browser Location API. screw URL, URN and URI.

• location.origin: https://www.somedomain.com
• location.pathname: /path/to/the/endpoint

Sundial

That doesn't explain the difference between the different terms in any way – Hunsaker

Answer 32

Don't forget URNs. URIs and URLs are both URNs. URLs have a location:

URI: foo
URL: http://some.domain.com/foo
URL: http://some.domain.com:8080/foo
URL: ftp://some.domain.com/foo

They're all URNs.

Geilich

Those are all URN's? I'm still unsure of the semantics of each but, syntactically, I thought that a URN has no slashes (//). See en.wikipedia.org/wiki/Uniform_Resource_Identifier Or is the lack of // just the typical case and not a syntactic requirement of a URN? – Protector