>perl -wMstrict -le
"use Regexp::Common qw(net);
 my $IPv4 = qr{ (?<! \d) $RE{net}{IPv4} (?! \d) }xms;
 ;;
 my @strs = (
   'permit ip host 10.11.1.1 192.168.100.0 0.0.0.255',
   'permit ip 10.11.1.0  0.0.0.255 192.168.100.0 0.0.0.255',
   );
 ;;
 for my $s (@strs) {
   print qq{from: '$s'};
   while ($s =~ m{ (?<! host) \s+ ($IPv4) \s+ ($IPv4) }xmsg) {
     print qq{IP pair: '$1' '$2'};
     }
   }
"
from: 'permit ip host 10.11.1.1 192.168.100.0 0.0.0.255'
IP pair: '192.168.100.0' '0.0.0.255'
from: 'permit ip 10.11.1.0  0.0.0.255 192.168.100.0 0.0.0.255'
IP pair: '10.11.1.0' '0.0.0.255'
IP pair: '192.168.100.0' '0.0.0.255'