206 Partial Content

HTTP 206 Partial Content is returned when a client asks for only part of a resource using a Range header and the server honors that request instead of sending the whole thing. It is the backbone of resumable downloads and media streaming: a video player can request just the next chunk of a file, a download manager can resume an interrupted transfer from the last byte it received, and a PDF viewer can fetch only the pages currently in view. The response includes a Content-Range header describing which bytes were returned and out of what total size, along with the partial payload itself.

For a range request to succeed with 206, the server must support byte-range requests on that resource, typically advertised in advance via an Accept-Ranges: bytes header on the full response, and the resource must not have changed between the original download and the resumed one. Servers validate this with conditional headers like If-Range, comparing an ETag or Last-Modified value; if the resource changed, they should return the full 200 response instead of a now-inconsistent partial one. A client can also request multiple non-contiguous ranges in one request, in which case the server returns a multipart/byteranges response body.

CDNs, static file servers, and object storage services widely support range requests for large files, since they let clients parallelize downloads across multiple ranges or resume after a network interruption without re-transferring already-received bytes. Media streaming in particular relies heavily on 206, since browsers issue range requests as users seek through a video or audio track rather than downloading the entire file up front. Misconfigured servers that ignore Range headers and always return 200 with the full body silently break resumable downloads and seeking.