What is a Range header?
Requests only part of a resource, e.g. bytes=0-999, used for resumable downloads and video seeking.
Range is what lets a visitor scrub through a video or resume a large download on a website instead of re-fetching the whole file from the start every time. Support for it needs to be tested deliberately - a monitor that only ever sends full GET requests will never notice that partial-content support has silently broken, even though it's core to how media-heavy pages actually behave for real visitors.
Common use case
A video player seeking to the halfway point of a file sends Range: bytes=5000000- to fetch only the remaining portion, instead of downloading and discarding everything before it.
Example
Range: bytes=0-1023History
Part of the original HTTP/1.1 specification, RFC 2068 (1997), alongside 206 Partial Content and Accept-Ranges.
Did you know?
It also supports multiple, non-contiguous ranges in a single request, which the server returns as a multipart response - a feature almost no real-world client actually uses.