mask!

# File lib/ipaddr.rb, line 496
  def mask!(mask)
    case mask
    when String
      if mask =~ /\A\d+\z/
        prefixlen = mask.to_i
      else
        m = IPAddr.new(mask)
        if m.family != @family
          raise InvalidPrefixError, "address family is not same"
        end
        @mask_addr = m.to_i
        n = @mask_addr ^ m.instance_variable_get(:@mask_addr)
        unless ((n + 1) & n).zero?
          raise InvalidPrefixError, "invalid mask #{mask}"
        end
        @addr &= @mask_addr
        return self
      end
    else
      prefixlen = mask
    end
    case @family
    when Socket::AF_INET
      if prefixlen < 0 || prefixlen > 32
        raise InvalidPrefixError, "invalid length"
      end
      masklen = 32 - prefixlen
      @mask_addr = ((IN4MASK >> masklen) << masklen)
    when Socket::AF_INET6
      if prefixlen < 0 || prefixlen > 128
        raise InvalidPrefixError, "invalid length"
      end
      masklen = 128 - prefixlen
      @mask_addr = ((IN6MASK >> masklen) << masklen)
    else
      raise AddressFamilyError, "unsupported address family"
    end
    @addr = ((@addr >> masklen) << masklen)
    return self
  end