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

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Re-explain

These are related but distinct concepts:

URI (Uniform Resource Identifier):

URL (Uniform Resource Locator):

URN (Uniform Resource Name):

In REST APIs, we primarily work with URLs to locate and access resources over HTTP.

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Interview lens How interviewers actually use this question

Walk me through the parts of https://api.example.com:8443/users/123?active=true#bio. Name the scheme, host, port, path, query and fragment. A strong answer notes that the client never sends the fragment in the HTTP request.
When does a value belong in the URL path, and when in the query string? The path identifies the resource or its place in a hierarchy, like an ID. The query refines the result with filters, sort order or a page number.
Why can't a client fetch urn:isbn:0451450523 directly, and how would you make it fetchable? A URN has no host and no access method. Something has to resolve the name to a location, such as a catalog service that maps ISBNs to URLs.
Which characters must be percent-encoded in a URL, and what breaks if you forget? Reserved characters like ?, #, &, / and spaces. An unencoded & splits one query value into two parameters, and a # cuts off the rest as a fragment.

URI is the umbrella term for any string that identifies a resource, and URLs and URNs are two kinds of URI.
A URL says where the resource lives and how to reach it, so it has a scheme like https, a domain and a path, for example https://api.example.com/users/123.
A URN names a resource inside a namespace without saying where it is. urn:isbn:0451450523 names a book, and that name doesn't change no matter who holds a copy.
So every URL and every URN is a URI, but not every URI is a URL. You can't send an HTTP request to an ISBN.
In REST APIs we work with URLs almost all the time, because the client needs a location it can send HTTP requests to.
In practice the line is blurry, since the URI spec lets one identifier be both a name and a locator. I say URI when I'm being precise and URL for anything I can fetch.

Says URI and URL are the same thing Every URL is a URI, but not every URI is a URL. Say that URI is the superset, then show a URN you can't fetch to prove it.
Says a URN tells you where the resource is A URN doesn't depend on location. urn:isbn names a book but has no host or protocol, so something else has to resolve it to a place.
Recites the acronyms with no examples Definitions alone blur together. One example per term, such as an https URL and a urn:isbn, makes the difference clear in seconds.
Claims URLs only use http or https ftp: and file: URLs also locate resources. The scheme names the access method, and HTTP is simply the one REST APIs use.

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