C# Signature:

[DllImport("iphlpapi.dll", CharSet=CharSet.Ansi)]
public static extern int GetAdaptersInfo(IntPtr pAdapterInfo, ref Int64 pBufOutLen);

VB Signature:

<DllImport("iphlpapi.dll", EntryPoint:="GetAdaptersInfo", CharSet:=CharSet.Ansi)> _
Private Shared Function GetAdaptersInfo( _
     ByVal pAdapterInfo As IntPtr, _
     ByRef pBufOutLen As UInt64) As Int32
End Function

Sample Code:

Public Sub GetAdapters()
     Dim structSize As Int32 = Marshal.SizeOf(GetType(IP_ADAPTER_INFO))
     Dim pArray As IntPtr = Marshal.AllocHGlobal(structSize)

     Dim len As UInt64 = Convert.ToUInt64(structSize)
     Dim ret As Int32 = GetAdaptersInfo(pArray, len)

     If ret = 111 Then
     ' Buffer was too small, reallocate the correct size for the buffer.
     pArray = Marshal.ReAllocHGlobal(pArray, New IntPtr(Convert.ToInt64(len)))
     ret = GetAdaptersInfo(pArray, len)
     End If

     If ret = 0 Then
     ' Call succeeded
     Dim pEntry As IntPtr = pArray
     Do
        ' Retrieve the adapter info from the memory address.
         Dim Entry As IP_ADAPTER_INFO = CType(Marshal.PtrToStructure(pEntry, GetType(IP_ADAPTER_INFO)), IP_ADAPTER_INFO)
         pEntry = Entry.Next
     Loop Until IntPtr.op_Equality(pEntry, IntPtr.Zero)
     End If

     Marshal.FreeHGlobal(pArray)
End Sub

C# Sample Code:

        const int MAX_ADAPTER_DESCRIPTION_LENGTH = 128;
        const int ERROR_BUFFER_OVERFLOW = 111;
        const int MAX_ADAPTER_NAME_LENGTH = 256;
        const int MAX_ADAPTER_ADDRESS_LENGTH = 8;
        const int MIB_IF_TYPE_OTHER = 1;
        const int MIB_IF_TYPE_ETHERNET = 6;
        const int MIB_IF_TYPE_TOKENRING = 9;
        const int MIB_IF_TYPE_FDDI = 15;
        const int MIB_IF_TYPE_PPP = 23;
        const int MIB_IF_TYPE_LOOPBACK = 24;
        const int MIB_IF_TYPE_SLIP = 28;

public static void GetAdapters()
{
   long structSize = Marshal.SizeOf( typeof( IP_ADAPTER_INFO ) );
   IntPtr pArray = Marshal.AllocHGlobal( new IntPtr(structSize) );

   int ret = GetAdaptersInfo( pArray, ref structSize );

   if (ret == ERROR_BUFFER_OVERFLOW ) // ERROR_BUFFER_OVERFLOW == 111
   {
     // Buffer was too small, reallocate the correct size for the buffer.
     pArray = Marshal.ReAllocHGlobal( pArray, new IntPtr (structSize) );

     ret = GetAdaptersInfo( pArray, ref structSize );
   } // if

   if ( ret == 0 )
   {
     // Call Succeeded
     IntPtr pEntry = pArray;

     do
     {
       // Retrieve the adapter info from the memory address
       IP_ADAPTER_INFO entry = (IP_ADAPTER_INFO)Marshal.PtrToStructure( pEntry, typeof( IP_ADAPTER_INFO ));

       // ***Do something with the data HERE!***
       Console.WriteLine("\n");
       Console.WriteLine( "Index: {0}", entry.Index.ToString() );

       // Adapter Type
       string tmpString = string.Empty;
       switch( entry.Type )
       {
         case MIB_IF_TYPE_ETHERNET  : tmpString = "Ethernet";  break;
         case MIB_IF_TYPE_TOKENRING : tmpString = "Token Ring"; break;
         case MIB_IF_TYPE_FDDI      : tmpString = "FDDI"; break;
         case MIB_IF_TYPE_PPP       : tmpString = "PPP"; break;
         case MIB_IF_TYPE_LOOPBACK  : tmpString = "Loopback"; break;
         case MIB_IF_TYPE_SLIP      : tmpString = "Slip"; break;
         default                    : tmpString = "Other/Unknown"; break;
       } // switch
       Console.WriteLine( "Adapter Type: {0}", tmpString );

       Console.WriteLine( "Name: {0}", entry.AdapterName );
       Console.WriteLine( "Desc: {0}\n", entry.AdapterDescription );

       Console.WriteLine( "DHCP Enabled: {0}", ( entry.DhcpEnabled == 1 ) ? "Yes" : "No" );

       if (entry.DhcpEnabled == 1)
       {
         Console.WriteLine( "DHCP Server : {0}", entry.DhcpServer.IpAddress.Address );

         // Lease Obtained (convert from "time_t" to C# DateTime)
         DateTime pdatDate = new DateTime(1970, 1, 1).AddSeconds( entry.LeaseObtained ).ToLocalTime();
         Console.WriteLine( "Lease Obtained: {0}", pdatDate.ToString() );

         // Lease Expires (convert from "time_t" to C# DateTime)
         pdatDate = new DateTime(1970, 1, 1).AddSeconds( entry.LeaseExpires ).ToLocalTime();
         Console.WriteLine( "Lease Expires : {0}\n", pdatDate.ToString() );
       } // if DhcpEnabled

       Console.WriteLine( "IP Address     : {0}", entry.IpAddressList.IpAddress.Address );
       Console.WriteLine( "Subnet Mask    : {0}", entry.IpAddressList.IpMask.Address );
       Console.WriteLine( "Default Gateway: {0}", entry.GatewayList.IpAddress.Address );

       // MAC Address (data is in a byte[])
       tmpString = string.Empty;
       for (int i = 0; i < entry.Address.Length - 1; i++)
       {
         tmpString += string.Format("{0:X2}-", entry.Address[i]);
       }
       Console.WriteLine( "MAC Address    : {0}{1:X2}\n", tmpString, entry.Address[entry.Address.Length - 1] );

       Console.WriteLine( "Has WINS: {0}", entry.HaveWins ? "Yes" : "No" );
       if (entry.HaveWins)
       {
         Console.WriteLine( "Primary WINS Server  : {0}", entry.PrimaryWinsServer.IpAddress.Address );
         Console.WriteLine( "Secondary WINS Server: {0}", entry.SecondaryWinsServer.IpAddress.Address );
       } // HaveWins

       // Get next adapter (if any)
       pEntry = entry.Next;

     }
     while( pEntry != IntPtr.Zero );

     Marshal.FreeHGlobal(pArray);

   } // if
   else
   {
     Marshal.FreeHGlobal(pArray);
     throw new InvalidOperationException( "GetAdaptersInfo failed: " + ret );
   }

} // GetAdapters

Notes:

GetAdaptersInfo returns a pointer to an IP_ADAPTER_INFO linked list.

Tip

In .NET 2.0, this library (The IP Helper API) is encapsulated in the NetworkInformation namespace.

Does this requiere a new dll?