// will return an invalid value -1.
func PrefixFrom(ip Addr, bits int) Prefix {
	var bitsPlusOne uint8
	if !ip.isZero() && bits >= 0 && bits <= ip.BitLen() {
		bitsPlusOne = uint8(bits) + 1
	}
	return Prefix{
		ip:          ip.withoutZone(),
		bitsPlusOne: bitsPlusOne,
	}
}

// Addr returns p's IP address.
func (p Prefix) Addr() Addr { return p.ip }