What's the best way to convert from network bitcount to netmask?

Assuming 32-bit mask and 32-bit int.

int keepBits = 24; /* actually get it from somewhere else? */ int mask = (0xffffffff >> (32 - keepBits )) << (32 - keepBits); 

Note: this isn't necessarily the answer to the question "What's the best way to get the network mask for an interface?"

I always do it like that (in your case cidr = 24):

uint32_t ipv4Netmask; ipv4Netmask = UINT32_MAX; ipv4Netmask <<= 32 - cidr; ipv4Netmask = htonl(ipv4Netmask); 

This will only work with ipv4Netmask to be actually uint32_t, don't make it int, as int doesn't have to be 32 Bit on every system. The result is converted to network byte order, as that's what most system functions expect.

Note that this code will fail if cidr is zero as then the code would shift a 32 bit variable by 32 bit and, believe it or not, that is undefined behavior in C. One would expect the result to always be zero but the C standard says that this is not defined to begin with. If your CIDR can be zero (which would only be allowed in the any IP address placeholder 0.0.0.0/0), then the code must catch special case.

This is not a programming question, but in linux you can use whatmask.

whatmask 72.20.10.0/24 

returns

IP Entered = ..................: 72.20.10.0 CIDR = ........................: /24 Netmask = .....................: 255.255.255.0 Netmask (hex) = ...............: 0xffffff00 Wildcard Bits = ...............: 0.0.0.255 ------------------------------------------------ Network Address = .............: 72.20.10.0 Broadcast Address = ...........: 72.20.10.255 Usable IP Addresses = .........: 254 First Usable IP Address = .....: 72.20.10.1 Last Usable IP Address = ......: 72.20.10.254 

int keepbits = 24; int mask = keepbits > 0 ? 0x00 - (1<<(32 - keepbits)) : 0xFFFFFFFF; 

Here's a solution in VBScript, FWIW

option explicit 'whatmask 72.20.10.0/24 If WScript.Arguments.Unnamed.Count < 1 Then WScript.Echo "WhatMask xxx.xxx.xxx.xxx/xx" Wscript.Quit End If Dim sToFind Dim aParts Dim nSubnet sToFind = WScript.Arguments(0) aParts = Split( sToFind, "/", 2 ) nSubnet = aParts(1) if nSubnet < 1 or nSubnet > 32 then WScript.echo "Subnet out of range [1..32]" Wscript.quit end if Dim sBinary sBinary = String( nSubnet, "1") sBinary = sBinary & String( 32 - nSubnet, "0" ) wscript.echo "0x" & lcase( binary2hexadecimal( sBinary ) ) function binary2hexadecimal( sBin ) dim sSlice dim sResult dim i for i = 1 to len( sBin ) step 4 sSlice = mid( sBin, i, 4 ) sResult = sResult & hex( binary2decimal( sSlice ) ) next binary2hexadecimal = sResult end function function binary2decimal( sFourbits ) dim i dim bit dim nResult nResult = 0 for i = 4 to 1 step -1 bit = mid(sFourbits, i, 1 ) nResult = nResult * 2 + bit next binary2decimal = nResult end function 

From the command line

>whatmask.vbs 123.12.123.17/23 0xfffff700 

Be careful when you use the previous answers with code like:

0xFFFFFFFF << 32 - cidr 

or

-1 << 32 - cidr 

In C# at least, it will mask the shift count with 0x1F first. So, for a cidr with prefix 0 (ie the entire IPv4 address range):

int cidr=0; 0xFFFFFFFF << (32 - cidr) == 0xFFFFFFFF 

which is not what you want. Instead, you should use:

int cidr=0; (int)(0xFFFFFFFFL << (32 - cidr)) == 0 

/* C# version merging some of the other contributions and corrected for byte order. */ int cidr = 24; var ipv4Netmask = 0xFFFFFFFF; ipv4Netmask <<= 32 - cidr; byte[] bytes = BitConverter.GetBytes(ipv4Netmask); Array.Reverse(bytes); ipv4Netmask = BitConverter.ToUInt32(bytes, 0); // mask is now ready for use such as: var netmask = new IPAddress(ipv4Netmask); 

You could try something simple, like taking the bitcount and dividing by 4. That'd give you the leading F's in the mask. And then take the remainder and have a switch from 0 bits to 3 bits.