code: func ParseIPv4(s string) (IPv4, error) {
  	ip := net.ParseIP(s)
  	if ip != nil {
  		ip = ip.To4()
  	}
  	if ip == nil {
  		return 0, errors.Reason("invalid IPv4 address %q", s).Err()
  	}
  	return IPv4(binary.BigEndian.Uint32(ip)), nil
  }
docs: // ParseIPv4 returns an IPv4 address from the given string.