// IPv4 mask a.b.c.d.
func IPv4Mask(a, b, c, d byte) IPMask {
	p := make(IPMask, IPv4len)
	p[0] = a
	p[1] = b
	p[2] = c
	p[3] = d
	return p
}

// CIDRMask returns an [IPMask] consisting of 'ones' 1 bits
// followed by 0s up to a total length of 'bits' bits.
// For a mask of this form, CIDRMask is the inverse of [IPMask.Size].
func CIDRMask(ones, bits int) IPMask {
	if bits != 8*IPv4len && bits != 8*IPv6len {
		return nil

			maskfld := fields[1]
			maskfld = maskfld[1:]
			pfxlen, _, ok := dtoi(maskfld)
			if !ok {
				return nil, errors.New("cannot parse network mask for interface: " + status)
			}
			var mask IPMask
			if ip.To4() != nil { // IPv4 or IPv6 IPv4-mapped address
				mask = CIDRMask(pfxlen-8*len(v4InV6Prefix), 8*IPv4len)
			}
			if ip.To16() != nil && ip.To4() == nil { // IPv6 address

				}

				if !isValidIPMask(mask) {
					return nil, nil, nil, nil, fmt.Errorf("x509: IP constraint contained invalid mask %x", mask)
				}

				ips = append(ips, &net.IPNet{IP: net.IP(ip), Mask: net.IPMask(mask)})

			case emailTag:
				constraint := string(value)
				if err := isIA5String(constraint); err != nil {
					return nil, nil, nil, nil, errors.New("x509: invalid constraint value: " + err.Error())
				}