C# Parse CIDR IP address and mask

ParseCIDR.cs

// quick and dirty, not very strict (accepts ipv6 on either side, and dotless ipv4)
static (IPAddress address, IPAddress mask) ParseCIDR (string str) {
    string[] parts = str.Split('/');
    if (parts.Length > 2)
        throw new FormatException("Invalid CIDR string.");
    else if (parts.Length == 1)
        return (IPAddress.Parse(parts[0]), IPAddress.Broadcast);
    else if (int.TryParse(parts[1], out int maskbits) && maskbits >= 0 && maskbits <= 32)
        return (IPAddress.Parse(parts[0]), new IPAddress(0xFFFFFFFFL & IPAddress.HostToNetworkOrder(unchecked((int)0x80000000) >> (maskbits - 1))));
    else
        return (IPAddress.Parse(parts[0]), IPAddress.Parse(parts[1]));
}