What is a Content-Range header?
Indicates where in the full resource a partial response body belongs, used with 206 Partial Content.
Content-Range accompanies a 206 Partial Content response to tell the client exactly which slice of the full resource it just received (e.g. bytes 500-999 of 5000), which is essential to correctly stitching together a resumed download or seeking within a video on a website. A mismatch between the Range request and the Content-Range response is a subtle bug that can silently corrupt downloaded files.
Common use case
A download manager reassembling a large file from several parallel partial downloads uses each response's Content-Range to know exactly where that chunk belongs in the final file.
Example
Content-Range: bytes 200-999/1000History
Part of the original HTTP/1.1 specification, RFC 2068 (1997), alongside Range and Accept-Ranges.
Did you know?
It can also appear on a 416 Range Not Satisfiable response using an asterisk in place of the range, telling the client the resource's total size without granting the specific range it asked for.