code: func NodeIdSecure(id [20]byte, ip net.IP) bool {
  	if isLocalNetwork(ip) {
  		return true
  	}
  	if ip4 := ip.To4(); ip4 != nil {
  		ip = ip4
  	}
  	crc := crcIP(ip, id[19])
  	if id[0] != byte(crc>>24&0xff) {
  		return false
  	}
  	if id[1] != byte(crc>>16&0xff) {
  		return false
  	}
  	if id[2]&0xf8 != byte(crc>>8&0xf8) {
  		return false
  	}
  	return true
  }
docs: // Returns whether the node ID is considered secure. The id is the 20 raw
  // bytes. http://www.libtorrent.org/dht_sec.html