Fastest way to skip lines while parsing files in Ruby?

I tried searching for this, but couldn't find much. It seems like something that's probably been asked before (many times?), so I apologize if that's the case.

I was wondering what the fastest way to parse certain parts of a file in Ruby would be. For example, suppose I know the information I want for a particular function is between lines 500 and 600 of, say, a 1000 line file. (obviously this kind of question is geared toward much large files, I'm just using those smaller numbers for the sake of example), since I know it won't be in the first half, is there a quick way of disregarding that information?

Currently I'm using something along the lines of:

while  buffer = file_in.gets and file_in.lineno <600
  next unless file_in.lineno > 500
  if buffer.chomp!.include? some_string
    do_func_whatever
  end
end

It works, but I just can't help but think it could work better.

I'm very new to Ruby and am interested in learning new ways of doing things in it.