You left out the crucial bit. Is someone actually getting 416 errors, or are they walking in off the street asking for the named file?
It's hardly your everyday error, so you wouldn't expect to find a custom error message. Did it start out as a failed 206?
Is that the actual IP? Are they all coming in from China? Don't know about the rest of youse, but I block them on sight-- especially when they're in back-to-back /11 /12 /13 ranges, as this one is.
for those who had to go look it up [w3.org] (as I did):
10.4.17 416 Requested Range Not Satisfiable
A server SHOULD return a response with this status code if a request included a Range request-header field (section 14.35 [w3.org]), and none of the range-specifier values in this field overlap the current extent of the selected resource, and the request did not include an If-Range request-header field. (For byte-ranges, this means that the first- byte-pos of all of the byte-range-spec values were greater than the current length of the selected resource.)
The next one, 417, is "Expectation Failed". If only real life were more like the Internet! ;)