Ok I can't seem to figure this out: given the following:
IP address = 192.168.1.0
Subnetmask = 255.255.255.240
Using c#, how do I calculate the CIDR notation 192.168.1.0/28 ? Is there an easy way to achieve this? Am I missing something?
Thanks!
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Jon Seigel
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RoelF
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7 Answers
7
256 - 240 = 16 = 2**4, 32 - 4 = 28
It is not really a C# question.
To get a net address from an IP and mask you can apply bytewise and to the IP and mask. You can get bytes from a string using IPAddress.Parse() and IPAddress.GetAddressBytes().
wRAR
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Thanks for the info. I know I can manipulate string to ip address and bytes with the IPAddress class. I also understand the logic behind the subnet/broadcast/cidr calculations etc... I'm just having trouble putting this into a simple c# method which gives me `26` when I input `255.255.255.192` – RoelF Mar 24 '10 at 13:42
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I know. That was not the question. Nevermind I figured it out. – RoelF Mar 24 '10 at 14:13
7
I had to do the same thing, no new info but this snippet may come in handy for the next person looking for a way to do this in C#. note that this method only counts the number of consecutive 1s, and leaves you the work of appending it to the IP.
public class IPAddressHelper
{
public static UInt32 SubnetToCIDR(string subnetStr)
{
IPAddress subnetAddress = IPAddress.Parse(subnetStr);
byte[] ipParts = subnetAddress.GetAddressBytes();
UInt32 subnet = 16777216 * Convert.ToUInt32(ipParts[0]) + 65536 * Convert.ToUInt32(ipParts[1]) + 256 * Convert.ToUInt32(ipParts[2]) + Convert.ToUInt32(ipParts[3]);
UInt32 mask = 0x80000000;
UInt32 subnetConsecutiveOnes = 0;
for (int i = 0; i < 32; i++)
{
if (!(mask & subnet).Equals(mask)) break;
subnetConsecutiveOnes++;
mask = mask >> 1;
}
return subnetConsecutiveOnes;
}
}
Yonatan Karni
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Keep it simple!
This works for IPv4 only, but since IPv6 does only support CIDR like /64 in fe80::1ff:fe23:4567:890a/64 calculations like that are unnecessary there.
All you need for an IPv4 network mask:
int cidr = Convert.ToString(mask.Address, 2).Count( o => o == '1');
Explanation based on the given example:
IPAddress mask = new IPAddress(new byte[] { 255, 255, 255, 240 });
// maskBinAsString = 11110000111101001111111111111111
string maskBinAsString = Convert.ToString(mask.Address, 2);
// cidr = 28
int cidr = Convert.ToString(mask.Address, 2).Count( o=> o == '1');
marsh-wiggle
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I don't have it as C# code but here is the answer in VB. Should not be to hard to convert.
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim someIP As Net.IPAddress = Net.IPAddress.Parse("192.168.1.10")
Dim someMASK As Net.IPAddress = Net.IPAddress.Parse("255.255.255.240")
Dim ipL As Long = IPtoLong(someIP)
Dim maskL As Long = IPtoLong(someMASK)
'Convert Mask to CIDR(1-30)
Dim oneBit As Long = &H80000000L
Dim CIDR As Integer = 0
For x As Integer = 31 To 0 Step -1
If (maskL And oneBit) = oneBit Then CIDR += 1 Else Exit For
oneBit = oneBit >> 1
Next
Dim answer As String = LongToIp(ipL And maskL) & " /" & CIDR.ToString
End Sub
Public Function IPtoLong(ByVal theIP As Net.IPAddress) As Long 'convert IP to number
Dim IPb() As Byte = theIP.GetAddressBytes 'get the octets
Dim addr As Long 'accumulator for address
For x As Integer = 0 To 3
addr = addr Or (CLng(IPb(x)) << (3 - x) * 8)
Next
Return addr
End Function
Public Function LongToIp(ByVal theIP As Long) As String 'convert number back to IP
Dim IPb(3) As Byte '4 octets
Dim addr As String = "" 'accumulator for address
Dim mask8 As Long = MaskFromCidr(8) 'create eight bit mask
For x = 0 To 3 'get the octets
IPb(x) = CByte((theIP And mask8) >> ((3 - x) * 8))
mask8 = mask8 >> 8
addr &= IPb(x).ToString & "." 'add current octet to string
Next
Return addr.TrimEnd("."c)
End Function
Private Function MaskFromCidr(ByVal CIDR As Integer) As Long
MaskFromCidr = CLng(2 ^ ((32 - CIDR)) - 1) Xor 4294967295L
End Function
dbasnett
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0
my solution, first parse to IPAddress:
var Subnetmask = "255.255.255.240";
IPAddress ip = IPAddress.Parse(Subnetmask);
then, check count of set bits in the mask ip:
var intAddress = (int)IPAddress.Address;
Console.WriteLine(NumberOfSetBits(intAddress)); //28
the function (from https://stackoverflow.com/a/12175897/1271037):
int NumberOfSetBits(int i)
{
i = i - ((i >> 1) & 0x55555555);
i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
return (((i + (i >> 4)) & 0x0F0F0F0F) * 0x01010101) >> 24;
}
0
I know it's a little late, but I took a swing converting dbasnett's answer VB.NET code above to C# (thanks to Telerik Code Converter) and put it up in a .NET Fiddle here and included my conversion below:
public static void Main()
{
System.Net.IPAddress someIP = System.Net.IPAddress.Parse("192.168.1.23");
System.Net.IPAddress someMASK = System.Net.IPAddress.Parse("255.255.255.128");
long ipL = IPtoLong(someIP);
long maskL = IPtoLong(someMASK);
// Convert Mask to CIDR(1-30)
long oneBit = 0x80000000L;
int CIDR = 0;
for (int x = 31; x >= 0; x += -1)
{
if ((maskL & oneBit) == oneBit)
CIDR += 1;
else
break;
oneBit = oneBit >> 1;
}
string answer = LongToIp(ipL & maskL) + "/" + CIDR.ToString();
Console.Out.WriteLine("woah woah we woah " + answer);
}
public static long IPtoLong(System.Net.IPAddress theIP) // convert IP to number
{
byte[] IPb = theIP.GetAddressBytes(); // get the octets
long addr = 0; // accumulator for address
for (int x = 0; x <= 3; x++) {
addr |= (System.Convert.ToInt64(IPb[x]) << (3 - x) * 8);
}
return addr;
}
public static string LongToIp(long theIP) // convert number back to IP
{
byte[] IPb = new byte[4]; // 4 octets
string addr = ""; // accumulator for address
long mask8 = MaskFromCidr(8); // create eight bit mask
for (var x = 0; x <= 3; x++) // get the octets
{
IPb[x] = System.Convert.ToByte((theIP & mask8) >> ((3 - x) * 8));
mask8 = mask8 >> 8;
addr += IPb[x].ToString() + "."; // add current octet to string
}
return addr.TrimEnd('.');
}
private static long MaskFromCidr(int CIDR)
{
return System.Convert.ToInt64(Math.Pow(2, ((32 - CIDR))) - 1) ^ 4294967295L;
}
Matt Plank
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This solution doesn't check the validity of the subnet mask. For example 255.255.255.253 will produce a result. Once we've found a zero in the subnet mask, we need to check that all remaining bits are also 0. – Andrew McLachlan Mar 28 '19 at 19:59
-1
Usage:
var cidrnetmask = MaskToCIDR(IPAddress.Parse("255.0.0.0").GetAddressBytes());
This works for IPv4. To support IPv6, one could expand the number of bytes but hopefully nobody will try and use old-style netmasks for IPv6 :o)
Method:
static int MaskToCIDR(byte[] bytes)
{
var b0 = bytes[0];
var b1 = bytes[1];
var b2 = bytes[2];
var b3 = bytes[3];
return
b3 != 0 ? (
(b3 & 0x01) != 0 ? 32 :
(b3 & 0x02) != 0 ? 31 :
(b3 & 0x04) != 0 ? 30 :
(b3 & 0x08) != 0 ? 29 :
(b3 & 0x10) != 0 ? 28 :
(b3 & 0x20) != 0 ? 27 :
(b3 & 0x40) != 0 ? 26 :
25 ) :
b2 != 0 ? (
(b2 & 0x01) != 0 ? 24 :
(b2 & 0x02) != 0 ? 23 :
(b2 & 0x04) != 0 ? 22 :
(b2 & 0x08) != 0 ? 21 :
(b2 & 0x10) != 0 ? 20 :
(b2 & 0x20) != 0 ? 19 :
(b2 & 0x40) != 0 ? 18 :
17 ) :
b1 != 0 ? (
(b1 & 0x01) != 0 ? 16 :
(b1 & 0x02) != 0 ? 15 :
(b1 & 0x04) != 0 ? 14 :
(b1 & 0x08) != 0 ? 13 :
(b1 & 0x10) != 0 ? 12 :
(b1 & 0x20) != 0 ? 11 :
(b1 & 0x40) != 0 ? 10 :
9 ) :
b0 != 0 ? (
(b0 & 0x01) != 0 ? 8 :
(b0 & 0x02) != 0 ? 7 :
(b0 & 0x04) != 0 ? 6 :
(b0 & 0x08) != 0 ? 5 :
(b0 & 0x10) != 0 ? 4 :
(b0 & 0x20) != 0 ? 3 :
(b0 & 0x40) != 0 ? 2 :
1 ) :
0;
}
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