base network adress calculation

> echo '192.168.12.43/27' | perl -pe '($_,$m)=split"/";$m*=-1;$a="";map{if(($m+=8)>0){$a.=$_>>$m<<$m}else{$a.=$_}$a.="."}split"\\.";$_=$a'

> 192.168.12.32

just so I don't forget :)