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?

Answer 1

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);     } }