Are byte ranges inclusive?

Yes. Both the first and last positions are inclusive and zero-based, so bytes=0-499 requests the first 500 bytes. The number of bytes returned is last − first + 1.

What is a suffix range?

A range written as bytes=-N (no first position) requests the final N bytes of the resource. For a 1000-byte file, bytes=-200 returns bytes 800 through 999. It is handy when you do not know the total size in advance.

When does a server return 416?

416 Range Not Satisfiable is returned when the first position is at or beyond the resource size, or every requested range is invalid. The response should include a Content-Range: bytes */ header stating the actual size.

What is Accept-Ranges for?

On a normal 200 response the server sends Accept-Ranges: bytes to advertise that it supports byte-range requests, or Accept-Ranges: none to say it does not. Clients use it to decide whether resumable or chunked downloads are possible.

Does this tool send my header anywhere?

No. Range parsing and resolution happen entirely in your browser with JavaScript. Nothing is uploaded, logged or stored.

HTTP Range Request Reference

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Fetching part of a resource

HTTP range requests let a client download just a slice of a resource — the foundation of resumable downloads, video seeking and parallel chunked transfers. The client asks with a Range header, the server answers with 206 Partial Content and a Content-Range, and advertises support up front with Accept-Ranges. This reference covers the full header set with a parser that resolves any Range value against a resource size.

How it works

A Range header names one or more byte ranges in the form first-last:

Range: bytes=0-499
Range: bytes=500-          (open-ended: 500 to the last byte)
Range: bytes=-200          (suffix: the final 200 bytes)
Range: bytes=0-499, 1000-1499   (multiple ranges → multipart/byteranges)

Positions are zero-based and inclusive. The server clamps a last value that runs past the end, treats a first at or beyond the size as unsatisfiable (416), and echoes the actual slice in Content-Range: bytes first-last/size. A single range yields one body; multiple ranges yield a multipart/byteranges response. An If-Range header makes the partial fetch conditional so a changed resource falls back to a full 200.

Tips and notes

Content-Length on a 206 is the size of the returned slice, not the whole file.
Use If-Range with the resource’s ETag to safely resume an interrupted download.
416 responses should carry Content-Range: bytes */<total> so clients learn the size.
Only the bytes unit is widely supported; servers ignore unknown range units.