code: func IPv4Range(block string) (IPv4, int64, error) {
  	ip, subnet, err := net.ParseCIDR(block)
  	if err != nil {
  		return 0, 0, errors.Reason("invalid CIDR block %q", block).Err()
  	}
  	ipv4 := ip.Mask(subnet.Mask).To4()
  	if ipv4 == nil {
  		return 0, 0, errors.Reason("invalid IPv4 CIDR block %q", block).Err()
  	}
  	ones, _ := subnet.Mask.Size()
  	return IPv4(binary.BigEndian.Uint32(ipv4)), 1 << uint32(32-ones), nil
  }
docs: // IPv4Range returns the starting address and length of the given IPv4 CIDR block.