That doesn't answer the question.

Yes the web server supports range requests. Yes it only returns 50kb.

But what mechanism is letting it scan to just those ranges in the binary file. Doesn't the file system make you seek to the starting block and then read from there?

The point is, while it looks very efficient, there might be a crap ton of IO going on for your little 50kb response.

EDIT: probably the webserver is doing an fseek(), and performance will vary based on file system impl. This is something I will need to benchmark.

For most filesystems it should be no issue to seek directly to a specific offset on a file, it's just like accessing an array, if you know where it starts and the offset you just need to add them and tell the disk to read that address.