The Content-Range entity-header is sent with a partial entity-body to specify where in the full entity-body the partial body should be inserted. It also indicates the total size of the full entity-body. When a server returns a partial response to a client, it must describe both the extent of the range covered by the response, and the length of the entire entity-body.
Content-Range = "Content-Range" ":" content-range-spec content-range-spec = byte-content-range-spec byte-content-range-spec = bytes-unit SP first-byte-pos "-" last-byte-pos "/" entity-length entity-length = 1*DIGIT
Unlike byte-ranges-specifier values, a byte-content-range-spec may only specify one range, and must contain absolute byte positions for both the first and last byte of the range.
A byte-content-range-spec whose last-byte-pos value is less than its first-byte-pos value, or whose entity-length value is less than or equal to its last-byte-pos value, is invalid. The recipient of an invalid byte-content-range-spec MUST ignore it and any content transferred along with it.
Examples of byte-content-range-spec values, assuming that the entity contains a total of 1234 bytes:
When an HTTP message includes the content of a single range (for example, a response to a request for a single range, or to a request for a set of ranges that overlap without any holes), this content is transmitted with a Content-Range header, and a Content-Length header showing the number of bytes actually transferred. For example,
HTTP/1.1 206 Partial content Date: Wed, 15 Nov 1995 06:25:24 GMT Last-modified: Wed, 15 Nov 1995 04:58:08 GMT Content-Range: bytes 21010-47021/47022 Content-Length: 26012 Content-Type: image/gif
When an HTTP message includes the content of multiple ranges (for example, a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart MIME message. The multipart MIME content-type used for this purpose is defined in this specification to be "multipart/byteranges". See appendix 19.2 for its definition.
A client that cannot decode a MIME multipart/byteranges message should not ask for multiple byte-ranges in a single request.
When a client requests multiple byte-ranges in one request, the server SHOULD return them in the order that they appeared in the request.
If the server ignores a byte-range-spec because it is invalid, the server should treat the request as if the invalid Range header field did not exist. (Normally, this means return a 200 response containing the full entity). The reason is that the only time a client will make such an invalid request is when the entity is smaller than the entity retrieved by a prior request.