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### Re: Determine which route to take

by rjt (Deacon)
 on Aug 03, 2013 at 02:15 UTC ( #1047672=note: print w/replies, xml ) Need Help??

in reply to Determine which route to take

I'd use a module, too, but if you want/need to know how the math works, it's actually pretty simple:

Update: Add this summary of the math:

Convert IP address (or route subnet) to integer:

```    my \$ip = (\$A << 24) + (\$B << 16) + (\$C << 8) + \$D;
# Do the same for each route address (\$net)

Convert /24 CIDR notation to 0xffffff00 netmask:

my \$mask = 0xffffffff ^ (1 << 32 - \$cidr) - 1;

Check if a destination IP matches a route:

Full example:

```#!/usr/bin/env perl

use 5.012;
use warnings FATAL => 'all';

# Use an array instead if order is important
my %routes = (
'192.168.0.1' => [ '192.168.0.0' => 24 ],
'10.10.10.1'  => [ '10.10.10.0'  => 24 ],
'default'     => [ '12.162.8.11' => 28 ],
);

printf "%15s -> %s\n", \$_, route(\$_)
for qw< 10.10.10.12 128.127.126.125 192.168.0.51
192.168.1.1 10.10.10.5 >;

sub ip {
\$_[0] =~ /^(\d+)\.(\d+)\.(\d+)\.(\d+)\$/ or die "Invalid IP";
die "Octet out of range" if (\$1 & \$2 & \$3 & \$4) > 255;
(\$1 << 24) + (\$2 << 16) + (\$3 << 8) + \$4
}

sub route {
my \$ip = ip(\$_[0]);
while (my (\$dest, \$route) = each %routes) {
my (\$net, \$cidr) = @\$route;
my \$mask = 0xffffffff ^ (1 << 32 - \$cidr) - 1;
return \$dest if (\$ip & \$mask) == ip(\$net);
}
return 'default';
}

Output:

```    10.10.10.12 -> 10.10.10.1
128.127.126.125 -> default
192.168.0.51 -> 192.168.0.1
192.168.1.1 -> default
10.10.10.5 -> 10.10.10.1

Replies are listed 'Best First'.
Re^2: Determine which route to take
by mhearse (Chaplain) on Aug 07, 2013 at 16:12 UTC
Excellent! Thanks so much. I could learn a lot by reading your code. Do you have a github page? Or webpage? With code? Also... where did you learn about bitshifting and netmasking? Can you recommend a book?

Thanks for the kind words, mhearse. I don't have a public GitHub or web presence, aside from my sundry answers on PerlMonks.

I'd better not answer that one directly. :-)

Have a look at Bitwise_operation, maybe Truth_table for some background. Once you have a decent handle on that, IP subnetting (which I've only just skimmed), looks like a decent intro to IPv4 math.

Can you recommend a book?

Unfortunately, my brain's bibliography is in serious need of trie optimization. Any relatively current text on IP networking will probably be a good start, though.

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