Just a little fun, assuming (gasp) sorted input.

#!/usr/bin/perl
use strict;
use warnings;
my $pip="";
my $ctr=0;
while (<DATA>)
{
 chomp;
 s/\.\d+$//;
 if ($pip ne $_ && $pip ne"")
 {
    print "$pip has $ctr entries\n";
    $ctr=1;
 }
 else
 {
    $ctr++;
 }
 $pip=$_;
}
__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
 192.168.10.46
 192.168.10.5
[download]

Enjoy,
dageek