A simple approach looks as follows:
var mac = 81952921372024;
mac.toString( 16 ) // "4a8926c44578"
.match( /.{1,2}/g ) // ["4a", "89", "26", "c4", "45", "78"]
.reverse() // ["78", "45", "c4", "26", "89", "4a"]
.join( ':' ) // "78:45:c4:26:89:4a"
> "78:45:c4:26:89:4a"
However I suggest putting additional 00
groups just for pathological cases when your integer is very short (i.e. mac = 150
):
var mac = 81952921372024;
new Array( 6 ).join( '00' ) // '000000000000'
.match( /../g ) // [ '00', '00', '00', '00', '00', '00' ]
.concat(
mac.toString( 16 ) // "4a8926c44578"
.match( /.{1,2}/g ) // ["4a", "89", "26", "c4", "45", "78"]
) // ["00", "00", "00", "00", "00", "00", "4a", "89", "26", "c4", "45", "78"]
.reverse() // ["78", "45", "c4", "26", "89", "4a", "00", "00", "00", "00", "00", "00", ]
.slice( 0, 6 ) // ["78", "45", "c4", "26", "89", "4a" ]
.join( ':' ) // "78:45:c4:26:89:4a"
> "78:45:c4:26:89:4a"