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.
– DennisFeb 8 '15 at 18:43

25

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."
– YojimboJul 20 '15 at 18:49

31 Answers
31

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 (actually not quite - see below), and all URNs are URIs - but URNs and URLs are different, so you can't say that all URIs are URLs.

EDIT: I had previously thought that all URLs are valid URIs, but as per comments:

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".

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.
– Mark CidadeOct 6 '08 at 21:38

12

Note that RFC 2396 has been obsoleted by RFC 3986 a long time ago (but that doesn't change the facts...)
– Julian ReschkeMar 11 '10 at 12:20

14

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".
– Adam GentMay 16 '13 at 0:47

5

@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?
– HubertJun 16 '13 at 23:32

7

@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.
– Andrew JankeMar 11 '14 at 5:00

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

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.

URNs are different from URLs in this rigid uniqueness constraint Does this mean that URLs don't uniquely identify a location?
– eugeneJul 11 '13 at 19:30

25

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).
– netjeffJul 26 '13 at 23:23

5

..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?
– ArneApr 28 '14 at 7:51

7

Is "earth128:Edward-de-Leau/6000000000569063853" (the unique me over multiple multiverse) a URN, a URL or a URI?
– edelwaterSep 20 '14 at 3:06

4

@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 :)
– user20358Apr 27 '15 at 10:57

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

http://example.com/mypage.html

ftp://example.com/download.zip

mailto:user@example.com

file:///home/user/file.txt

tel:1-888-555-5555

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: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/

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.

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.

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..."
– Stephen OstermillerJul 5 '15 at 2:11

3

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
– veeMay 22 '17 at 7:41

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!
– Phil SturgeonDec 31 '09 at 12:18

129

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.
– Jörg W MittagDec 31 '09 at 12:40

12

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.
– Phil SturgeonJan 11 '10 at 11:13

11

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.
– Andrew JankeMar 11 '14 at 5:33

30

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.
– ArtOfWarfareOct 6 '14 at 17:33

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
– ruvimDec 4 '14 at 14:57

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 and 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.

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

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.

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.
– Tim GautierJan 10 '18 at 17:28

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.

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.
– Chris CharabarukOct 6 '08 at 21:30

3

A URL locates a resource and is a kind of URI, which identifies a resource.
– Mark CidadeOct 6 '08 at 21:33

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.
– Stephen OstermillerApr 17 '15 at 10:11

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”).

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.

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".

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.
– ccjmneSep 6 '14 at 11:06

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.
– Stephen OstermillerApr 17 '15 at 10:03

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.

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.

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...)

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

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.)
– Stephen OstermillerApr 17 '15 at 10:07

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:fred@mail.org, 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:0E4FC272-5C02-11D9-B115-000A95B55BC8@stackoverflow.com 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.

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.
– Vegan SvDec 28 '15 at 0:54

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.

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

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