1:  # sub validateMask
2:  #
3:  #     takes a dotted-decimal IP mask
4:  #
5:  #     returns 1 if the mask is valid
6:  #     returns undef if not
7:  #
8:  
9:  sub validateMask {
10:   my $mask       = shift;
11:   my $net        = 0;
12:   return
12:     unless $mask =~/^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/;
12y:                                   # accept only 1 to 3 digit octets in above regex
14:   foreach ( split ( /\./, $mask ) ) {
15:     if (
16:     $net    == 1
17:     &&
18:     $_      != 0
19:     )                 { return; }
20:     if ( $_      > 255 ) { return; }
21:     if ( $_     == 255 ) { next;   }
22:     if (
22y1:   $_ % 8 != 0                       # if an octet mod 8 isn't 0, bad mask
22y2:   &&
22y3:   $_ != 252                         # 255 is valid octet, but octet mod 8 isn't 0
22y4:   )   { return; }
23:     $net = 1;
24:     next;
25:   }
26:   return 1;
27: }