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What is the difference between a URI, a URL, and a URN?
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What's the difference between an URI, URL and URN? I have read a lot of sites (even Wikipedia) but I don't understand it.
URI: http://www.foo.com/bar.html
URL: http://www.foo.com/bar.html
URN: bar.html
Is this correct?
A Uniform Resource Identifier (URI) is a string of characters used to identify a name or a resource on the Internet.
A URI identifies a resource either by location, or a name, or both. A URI has two specializations known as URL and URN.
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. A URL defines how the resource can be obtained. It does not have to be a HTTP URL (http://), a URL can also start with ftp:// or smb://, specifying the protocol that's used to get the resource.
A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) that uses the URN scheme, and does not imply availability of the identified resource. Both URNs (names) and URLs (locators) are URIs, and a particular URI may be both a name and a locator at the same time.
This diagram (source) visualizes the relationship between URI, URN, and URL:
The URNs are part of a larger Internet information architecture which is composed of URNs, URCs and URLs.
bar.html is not a URN. A URN is similar to a person's name, while a URL is like a street address. The URN defines something's identity, while the URL provides a location. Essentially URN vs. URL is "what" vs. "where". A URN has to be of this form <URN> ::= "urn:" <NID> ":" <NSS> where <NID> is the Namespace Identifier, and <NSS> is the Namespace Specific String.
To put it differently:
A URL is a URI that identifies a resource and also provides the means of locating the resource by describing the way to access it
A URL is a URI
A URI is not necessarily a URL
I'd say the only thing left to make it 100% clear would be to have an example of an URI that is not an URL. We can use the examples in RFC3986:
URL: ftp://ftp.is.co.za/rfc/rfc1808.txt
URL: http://www.ietf.org/rfc/rfc2396.txt
URL: ldap://[2001:db8::7]/c=GB?objectClass?one
URL: mailto:John.Doe#example.com
URL: news:comp.infosystems.www.servers.unix
URL: telnet://192.0.2.16:80/
URN (not URL): urn:oasis:names:specification:docbook:dtd:xml:4.1.2
URN (not URL): tel:+1-816-555-1212 (disputed, see comments)
Below I sum up Prateek Joshi's awesome explanation.
The theory:
URI (uniform resource identifier) identifies a resource (text document, image file, etc)
URL (uniform resource locator) is a subset of the URIs that include a network location
URN (uniform resource name) is a subset of URIs that include a name within a given space, but no location
That is:
And for an example:
Also, if you haven't already, I suggest reading Roger Pate's answer.
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:user#example.com
file:///home/user/file.txt
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 name. It always 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.
URI -- Uniform Resource Identifier
URIs encompasses both URLs, URNs, and other ways to indicate a resource.
An example of a URI that is neither a URL nor a URN would be a data URI such as data:,Hello%20World. It is not a URL or URN because the URI contains the data. It neither names it, nor tells you how to locate it over the network.
There are also uniform resource citations (URCs) that point to meta data about a document rather than to the document itself. An example of a URC would be an indicator for viewing the source code of a web page: view-source:http://example.com/. A URC is another type of URI that is neither URL nor URN.
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.
Can a URI be both a URL and a URN?
Good question. I've seen lots of places on the web that state this is true. I haven't been able to find any examples of something that is both a URL and a URN. I don't see how it is possible because a URN starts with urn: which is not a valid network protocol.
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.)
Is a tel: URI a URL or a URN?
For example tel:1-800-555-5555. It doesn't start with urn: and it has a protocol for reaching a resource over a network. It must be a URL.
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 URL and URI interchangeably. It is no longer useful to strictly segment URIs into different types such as URL, URN, and URC.
URI (Uniform Resource Identifier) according to Wikipedia:
a string of characters used to identify a resource.
URL (Uniform Resource Locator) is a URI that implies an interaction mechanism with resource. for example https://www.google.com specifies the use of HTTP as the interaction mechanism. Not all URIs need to convey interaction-specific information.
URN (Uniform Resource Name) is a specific form of URI that has urn as it's scheme. For more information about the general form of a URI refer to https://en.wikipedia.org/wiki/Uniform_Resource_Identifier#Syntax
IRI (International Resource Identifier) is a revision to the definition of URI that allows us to use international characters in URIs.
What is the difference between a URL, a URI, and a URN?
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.
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.
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: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 -- 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.
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.
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)
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.
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
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).
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):
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!
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.
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.
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.
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/).
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”).
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.
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.
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".
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
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.
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.
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.
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.
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.
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...)
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
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
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.
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.
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
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.