Replace DATA with <> or whatever file handle you love.

use strict;
use warnings;
my %ips = ();
map { $ips{ join('.',(split('\.',$_,4))[0..2] ) }++ } <DATA>;
print "$_ has $ips{$_} nodes\n" foreach sort keys %ips;


__DATA__
10.10.10.1
10.10.10.2
10.10.10.3
10.10.12.1
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