How to check if an IP address is within a particular subnet
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 (maskLength == 0)
{
return true;
}
if (maskLength < 0)
{
throw new NotSupportedException("A Subnetmask should not be less than 0.");
}
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. Reverse the BitArray to compare the bits of each byte in the right order.
var maskAddressBits = new BitArray(maskAddress.GetAddressBytes().Reverse().ToArray());
// And convert the IpAddress to a BitArray. Reverse the BitArray to compare the bits of each byte in the right order.
var ipAddressBits = new BitArray(address.GetAddressBytes().Reverse().ToArray());
var ipAddressLength = ipAddressBits.Length;
if (maskAddressBits.Length != ipAddressBits.Length)
{
throw new ArgumentException("Length of IP Address and Subnet Mask do not match.");
}
// Compare the prefix bits.
for (var i = ipAddressLength - 1; i >= ipAddressLength - maskLength; i--)
{
if (ipAddressBits[i] != maskAddressBits[i])
{
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")]
[InlineData("192.168.5.85/0", "0.0.0.0")]
[InlineData("192.168.5.85/0", "255.255.255.255")]
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")]
[InlineData("2001:db8:abcd:5678::0/53", "2001:0db8:abcd:5000:0000:0000:0000:0000")]
[InlineData("2001:db8:abcd:5678::0/53", "2001:0db8:abcd:57ff:ffff:ffff:ffff:ffff")]
[InlineData("2001:db8:abcd:0012::0/0", "::")]
[InlineData("2001:db8:abcd:0012::0/0", "ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff")]
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")]
[InlineData("2001:db8:abcd:5678::0/53", "2001:0db8:abcd:4999:0000:0000:0000:0000")]
[InlineData("2001:db8:abcd:5678::0/53", "2001:0db8:abcd:5800:0000:0000:0000:0000")]
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 ;)
Take a look at IP Address Calculations with C# on MSDN blogs. It contains an extension method (IsInSameSubnet) that should meet your needs as well as some other goodies.
public static class IPAddressExtensions
{
public static IPAddress GetBroadcastAddress(this IPAddress address, IPAddress subnetMask)
{
byte[] ipAdressBytes = address.GetAddressBytes();
byte[] subnetMaskBytes = subnetMask.GetAddressBytes();
if (ipAdressBytes.Length != subnetMaskBytes.Length)
throw new ArgumentException("Lengths of IP address and subnet mask do not match.");
byte[] broadcastAddress = new byte[ipAdressBytes.Length];
for (int i = 0; i < broadcastAddress.Length; i++)
{
broadcastAddress[i] = (byte)(ipAdressBytes[i] | (subnetMaskBytes[i] ^ 255));
}
return new IPAddress(broadcastAddress);
}
public static IPAddress GetNetworkAddress(this IPAddress address, IPAddress subnetMask)
{
byte[] ipAdressBytes = address.GetAddressBytes();
byte[] subnetMaskBytes = subnetMask.GetAddressBytes();
if (ipAdressBytes.Length != subnetMaskBytes.Length)
throw new ArgumentException("Lengths of IP address and subnet mask do not match.");
byte[] broadcastAddress = new byte[ipAdressBytes.Length];
for (int i = 0; i < broadcastAddress.Length; i++)
{
broadcastAddress[i] = (byte)(ipAdressBytes[i] & (subnetMaskBytes[i]));
}
return new IPAddress(broadcastAddress);
}
public static bool IsInSameSubnet(this IPAddress address2, IPAddress address, IPAddress subnetMask)
{
IPAddress network1 = address.GetNetworkAddress(subnetMask);
IPAddress network2 = address2.GetNetworkAddress(subnetMask);
return network1.Equals(network2);
}
}