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
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.
The spec does not explicitly forbid or discourage it, so I would tend to say it is allowed.
Microsoft sees it the same way (I can hear murmuring in the audience), they state in the MSDN article about the DELETE Method of ADO.NET Data Services Framework:
If a DELETE request includes an entity body, the body is ignored [...]
Additionally here is what RFC2616 (HTTP 1.1) has to say in regard to requests:
- an entity-body is only present when a message-body is present (section 7.2)
- the presence of a message-body is signaled by the inclusion of a
Content-Length
or Transfer-Encoding
header (section 4.3)
- a message-body must not be included when the specification of the request method does not allow sending an entity-body (section 4.3)
- an entity-body is explicitly forbidden in TRACE requests only, all other request types are unrestricted (section 9, and 9.8 specifically)
For responses, this has been defined:
- whether a message-body is included depends on both request method and response status (section 4.3)
- a message-body is explicitly forbidden in responses to HEAD requests (section 9, and 9.4 specifically)
- a message-body is explicitly forbidden in 1xx (informational), 204 (no content), and 304 (not modified) responses (section 4.3)
- all other responses include a message-body, though it may be of zero length (section 4.3)
Best Answer
This turned out to be a GET request size limit on our internal firewall proxy server. We were able to reduce the size at which we switch from a GET to a POST request in our code to avoid the limit.