// Contains reports whether the network p includes ip.
//
// An IPv4 address will not match an IPv6 prefix.
// An IPv4-mapped IPv6 address will not match an IPv4 prefix.
// A zero-value IP will not match any prefix.
// If ip has an IPv6 zone, Contains returns false,
// because Prefixes strip zones.
func (p Prefix) Contains(ip Addr) bool {
	if !p.IsValid() || ip.hasZone() {
		return false
	}
	if f1, f2 := p.ip.BitLen(), ip.BitLen(); f1 == 0 || f2 == 0 || f1 != f2 {
		return false
	}
	if ip.Is4() {
		// xor the IP addresses together; mismatched bits are now ones.
		// Shift away the number of bits we don't care about.
		// Shifts in Go are more efficient if the compiler can prove
		// that the shift amount is smaller than the width of the shifted type (64 here).
		// We know that p.bits is in the range 0..32 because p is Valid;
		// the compiler doesn't know that, so mask with 63 to help it.
		// Now truncate to 32 bits, because this is IPv4.
		// If all the bits we care about are equal, the result will be zero.
		return uint32((ip.addr.lo^p.ip.addr.lo)>>((32-p.Bits())&63)) == 0
	} else {
		// xor the IP addresses together.
		// Mask away the bits we don't care about.
		// If all the bits we care about are equal, the result will be zero.
		return ip.addr.xor(p.ip.addr).and(mask6(p.Bits())).isZero()
	}
}

Warm-up