If what you mean by node is unique values:

use strict;
use warnings;

my %ips;
my @array = qw ( 
  10.0.0.1 10.0.0.2 10.0.0.3 
  192.168.1.5 192.168.1.6 
  192.168.10.3 192.168.10.4 
  192.168.10.5 10.0.0.1);

foreach (@array) { 
  $ips{$1}{$2} = undef if /([\d.]+)\.(\d+)$/;
}

foreach ( keys %ips ) { 
  my $keys = keys %{$ips{$_}}; 
  print "$_: $keys nodes\n"  
}
[download]

If by nodes you meant values appearing regardless of possible duplication:

use strict;
use warnings;

my %ips;
my @array = qw ( 
  10.0.0.1 10.0.0.2 10.0.0.3 
  192.168.1.5 192.168.1.6 
  192.168.10.3 192.168.10.4 
  192.168.10.5 10.0.0.1);

foreach (@array) { 
  $ips{$1}++ if /([\d.]+)\.(\d+)$/;
}

foreach ( keys %ips ) { 
  print "$_: $ips{$_} nodes\n"  
}
[download]

(Note the different value of 10.0.0 caused by the extra 10.0.0.1 in the array)

-enlil