use 5.010;
use strict;
use warnings;

my %ips;
while (<$in_fh>)
{
	if (/^(\d+)\.(\d+)\.(\d+)\.(\d+)/)
	{
		my $ip     = "$1.$2.$3.$4";
		my $classc = "$1.$2.$3";
		push @{ $ips{$classc} }, $ip;
	}
}

foreach my $classc (sort keys %ips)
{
	say $ips{$classc}[rand @{$ips{$classc}}];
}