c# - How to check if an IP address is within a particular subnet

Question

I have a subnet in the format 10.132.0.0/20 and an IP address from the ASP.Net request object.

Is there a .NET framework function to check to see if the IP address is within the given subnet?

If not, how can it be done? Bit manipulation, I guess?

question from:https://stackoverflow.com/questions/1499269/how-to-check-if-an-ip-address-is-within-a-particular-subnet

Answer 1

Using the answers from Thomas and Chris together with Ciscos Subnetting Examples I finally got something to work for IPv4 and IPv6 if you use the CIDR notation (IPAddress/PrefixLength). My IPv6-Implementation might be a bit too straight forward but as there is no UInt128-datatype I couldn't adapt Thomas's solution. Here is the code that seems to work well:

public static bool IsInSubnet(this IPAddress address, string subnetMask)
        {
            var slashIdx = subnetMask.IndexOf("/");
            if (slashIdx == -1)
            { // We only handle netmasks in format "IP/PrefixLength".
                throw new NotSupportedException("Only SubNetMasks with a given prefix length are supported.");
            }

            // First parse the address of the netmask before the prefix length.
            var maskAddress = IPAddress.Parse(subnetMask.Substring(0, slashIdx));

            if (maskAddress.AddressFamily != address.AddressFamily)
            { // We got something like an IPV4-Address for an IPv6-Mask. This is not valid.
                return false;
            }

            // Now find out how long the prefix is.
            int maskLength = int.Parse(subnetMask.Substring(slashIdx + 1));

            if (maskAddress.AddressFamily == AddressFamily.InterNetwork)
            {
                // Convert the mask address to an unsigned integer.
                var maskAddressBits = BitConverter.ToUInt32(maskAddress.GetAddressBytes().Reverse().ToArray(), 0);

                // And convert the IpAddress to an unsigned integer.
                var ipAddressBits = BitConverter.ToUInt32(address.GetAddressBytes().Reverse().ToArray(), 0);

                // Get the mask/network address as unsigned integer.
                uint mask = uint.MaxValue << (32 - maskLength);

                // https://stackoverflow.com/a/1499284/3085985
                // Bitwise AND mask and MaskAddress, this should be the same as mask and IpAddress
                // as the end of the mask is 0000 which leads to both addresses to end with 0000
                // and to start with the prefix.
                return (maskAddressBits & mask) == (ipAddressBits & mask);
            }

            if (maskAddress.AddressFamily == AddressFamily.InterNetworkV6)
            {
                // Convert the mask address to a BitArray.
                var maskAddressBits = new BitArray(maskAddress.GetAddressBytes());

                // And convert the IpAddress to a BitArray.
                var ipAddressBits = new BitArray(address.GetAddressBytes());

                if (maskAddressBits.Length != ipAddressBits.Length)
                {
                    throw new ArgumentException("Length of IP Address and Subnet Mask do not match.");
                }

                // Compare the prefix bits.
                for (int maskIndex = 0; maskIndex < maskLength; maskIndex++)
                {
                    if (ipAddressBits[maskIndex] != maskAddressBits[maskIndex])
                    {
                        return false;
                    }
                }

                return true;
            }

            throw new NotSupportedException("Only InterNetworkV6 or InterNetwork address families are supported.");
        }

And this are the XUnit tests I tested it with:

public class IpAddressExtensionsTests
    {
        [Theory]
        [InlineData("192.168.5.85/24", "192.168.5.1")]
        [InlineData("192.168.5.85/24", "192.168.5.254")]
        [InlineData("10.128.240.50/30", "10.128.240.48")]
        [InlineData("10.128.240.50/30", "10.128.240.49")]
        [InlineData("10.128.240.50/30", "10.128.240.50")]
        [InlineData("10.128.240.50/30", "10.128.240.51")]
        public void IpV4SubnetMaskMatchesValidIpAddress(string netMask, string ipAddress)
        {
            var ipAddressObj = IPAddress.Parse(ipAddress);
            Assert.True(ipAddressObj.IsInSubnet(netMask));
        }

        [Theory]
        [InlineData("192.168.5.85/24", "192.168.4.254")]
        [InlineData("192.168.5.85/24", "191.168.5.254")]
        [InlineData("10.128.240.50/30", "10.128.240.47")]
        [InlineData("10.128.240.50/30", "10.128.240.52")]
        [InlineData("10.128.240.50/30", "10.128.239.50")]
        [InlineData("10.128.240.50/30", "10.127.240.51")]
        public void IpV4SubnetMaskDoesNotMatchInvalidIpAddress(string netMask, string ipAddress)
        {
            var ipAddressObj = IPAddress.Parse(ipAddress);
            Assert.False(ipAddressObj.IsInSubnet(netMask));
        }

        [Theory]
        [InlineData("2001:db8:abcd:0012::0/64", "2001:0DB8:ABCD:0012:0000:0000:0000:0000")]
        [InlineData("2001:db8:abcd:0012::0/64", "2001:0DB8:ABCD:0012:FFFF:FFFF:FFFF:FFFF")]
        [InlineData("2001:db8:abcd:0012::0/64", "2001:0DB8:ABCD:0012:0001:0000:0000:0000")]
        [InlineData("2001:db8:abcd:0012::0/64", "2001:0DB8:ABCD:0012:FFFF:FFFF:FFFF:FFF0")]
        [InlineData("2001:db8:abcd:0012::0/128", "2001:0DB8:ABCD:0012:0000:0000:0000:0000")]
        public void IpV6SubnetMaskMatchesValidIpAddress(string netMask, string ipAddress)
        {
            var ipAddressObj = IPAddress.Parse(ipAddress);
            Assert.True(ipAddressObj.IsInSubnet(netMask));
        }

        [Theory]
        [InlineData("2001:db8:abcd:0012::0/64", "2001:0DB8:ABCD:0011:FFFF:FFFF:FFFF:FFFF")]
        [InlineData("2001:db8:abcd:0012::0/64", "2001:0DB8:ABCD:0013:0000:0000:0000:0000")]
        [InlineData("2001:db8:abcd:0012::0/64", "2001:0DB8:ABCD:0013:0001:0000:0000:0000")]
        [InlineData("2001:db8:abcd:0012::0/64", "2001:0DB8:ABCD:0011:FFFF:FFFF:FFFF:FFF0")]
        [InlineData("2001:db8:abcd:0012::0/128", "2001:0DB8:ABCD:0012:0000:0000:0000:0001")]
        public void IpV6SubnetMaskDoesNotMatchInvalidIpAddress(string netMask, string ipAddress)
        {
            var ipAddressObj = IPAddress.Parse(ipAddress);
            Assert.False(ipAddressObj.IsInSubnet(netMask));
        }
    }

As base for the tests I used Ciscos Subnetting Examples and IBMs IPV6 address examples.

I hope someone finds this helpful ;)