my @IP=qw(10.0.4.100 10.0.4.99 10.3.5.21 10.0.4.101 10.15.21.6);

my @sorted_IP=map{substr($_,4) }
              sort
              map{pack('C4',/(\d+)\.(\d+)\.(\d+)\.(\d+)/).$_}@IP;