[Networks] How to calculate a netmask

To say that I've been busy of late is somewhat of an understatement. Mind you, I've had time to create a presence over on StackOverflow, and to start "playing" MouseHunt on FaceBook.

I was responding to something over on StackOverflow: "What’s the best way to convert from network bitcount to netmask?" and added my two-bob to the mix with the following bit of VBScript.

option explicit 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

A few examples runs are as follows:

>whatmask 72.20.10.0/24 0xffffff00 >whatmask 191.33.12.0/17 0xffff1000 >whatmask 10.0.0.1/4 0xf0000000 >whatmask 10.0.0.1/3 0x70000000@text

It's been ages since I did the Cisco coursework (never did sit the exam) so I'm not sure if it's totally correct.

The binary2hexadecimal and binary2decimal functions were interesting to write as I was trying to find simpler, more lateral ways of doing things.

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

© Copyright Bruce M. Axtens, 2008