mustIP("::"),
	} {
		got := ip.Prev()
		if !got.isZero() {
			t.Errorf("IP(%v).Prev = %v; want zero", ip, got)
		}
	}

	var allFF [16]byte
	for i := range allFF {
		allFF[i] = 0xff
	}

	for _, ip := range []Addr{
		mustIP("255.255.255.255"),
		AddrFrom16(allFF),
	} {
		got := ip.Next()
		if !got.isZero() {
			t.Errorf("IP(%v).Next = %v; want zero", ip, got)
		}
	}
}

		return classBMask
	default:
		return classCMask
	}
}

func allFF(b []byte) bool {
	for _, c := range b {
		if c != 0xff {
			return false
		}
	}
	return true
}

// Mask returns the result of masking the IP address ip with mask.
func (ip IP) Mask(mask IPMask) IP {
	if len(mask) == IPv6len && len(ip) == IPv4len && allFF(mask[:12]) {
		mask = mask[12:]
	}