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Re: Algorithom to find overlaping subnets (Internet IPv4)

by rg0now (Chaplain)
on Sep 19, 2011 at 12:29 UTC ( #926729=note: print w/replies, xml ) Need Help??

in reply to Algorithom to find overlaping subnets (Internet IPv4)

50 thousand or so prefixes is not that many, so a quick and dirty O(n^2) compare-each-subnet-pair would do it based on, say, Net::IP overlap.

If speed is a concern, then I would go with your "build a tree of up to 32 levels" idea. Such a tree is called a prefix trie, and it is often used in IP routing tables. You can code your own or choose something from CPAN (say, Net::IPTrie or Tree::Trie).

Note, however, that checking overlaps is more difficult than your "inserting a network I find the space taken, I would have found an overlap" idea, because not just the node itself, but all its parents must be checked as well. In particular, you have an overlap either if the node corresponding to your subnet is taken or any of its parents is taken.

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Re^2: Algorithom to find overlaping subnets (Internet IPv4)
by chrestomanci (Priest) on Sep 20, 2011 at 13:47 UTC

    Thanks for your informative reply.

    If I had know that mod::Net::IPTrie or mod::Tree::Trie existed I would have use them, unfortunately I did not know they existed and did not know the terminology to search for them on CPAN, and I thought it would take to long to implement something myself.

    Instead I wrote some code using string representations of the binary bits in a database using DBIx::Class.

    My DBIC table defintion looks like this:

    __PACKAGE__->table('tblSubNet'); __PACKAGE__->add_columns( 'id' => { data_type=>'int', is_auto_increment=>1 + }, 'ip' => { data_type=>'varchar', size=>15 + }, 'mask' => { data_type=>'int' + }, 'name' => { data_type=>'varchar', size=>255, is_nullable=>1 + }, 'start' => { data_type=>'int' + }, # the IP address as A 32 bit number 'bitpattern' => { data_type=>'varchar', size=>32 + }, ); __PACKAGE__->set_primary_key('id'); __PACKAGE__->add_unique_constraint(['bitpattern']);

    Once I have populated the table of subnets, I then search it like this:

    my $order_by_size_rs = $all_subnets_rs->search({'source'=>$source},{'o +rder_by'=>{'-asc','mask'}}); while( my $network = $order_by_size_rs->next ) { my $overlaping_nets_rs = $net_rs->search({ 'mask' => { '>', $network->mask() }, 'bitpattern' => { 'like', $network->bitpattern().'%' }, }, { 'order_by'=>{'-asc',['mask','start']} }); if( $overlaping_nets_rs->count ) { printf "Subnet %s/%d (%s) has %d overlaps:\n", $network->ip, $network->mask, $network->name, $overlaping_nets_rs->count; ... # Code to add the overlaps to the report. } }

    Using this algorithm I was able to search through the 50_000 subnets searching for overlaps in about 10 minutes. (On a 3GHz Linux box).

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