One more for you:

 #!/usr/bin/perl

 use strict;
 use warnings;

 my %nodes;

 while ( <DATA> ) {
   chomp;
   s/\.[^\.]$//;
   $nodes{ $_ }++;
 }

 foreach ( sort keys %nodes ) {
   print "$_ $nodes{$_} nodes\n";
 }

 __DATA__
 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
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