Getting available CIDR netblocks in PHP - php

Question

Given a parent subnet and a number of "used" subnets, what would be the best possible method/algorithm to determine "unused" subnets?
For example, given:
$parent = '10.0.0.0/24';
$used = [
'10.0.0.0/29',
'10.0.0.8/29',
'10.0.0.144/29'
];
The program would need to come up with the smallest array of unused subnets:
$unused = [
'10.0.0.16/28',
'10.0.0.32/27',
'10.0.0.64/26',
'10.0.0.128/28',
'10.0.0.152/29',
'10.0.0.160/27',
'10.0.0.192/26'
];
I was planning on using the following pseudocode to achieve this:
create array $unused
add $parent to $unused
iterate through used subnets:
if $used in $unused (exact match):
remove from $unused
else
find subnet in $unused that contains $used
split into smaller subnets
However, I'm unable to figure out exactly how to do the split into smaller subnets part. If anyone could advise the best possible way to do this, or a better/more efficient method, it would be greatly appreciated!

Answer 1

One possible approach would be a form of recursive binary search.
A subnet is a range of IP addresses, so a first step would be developing functions to convert subnets to ranges and back again (in your example you've only shown IPv4 addresses, so I'm going to use integers to represent the ranges):
function subnetToRange($subnet)
{
list($ip_address, $mask_length) = explode('/', $subnet);
$ip = ip2long($ip_address);
$mask_right = pow(2, 32 - $mask_length) - 1;
$mask_left = 0xffffffff ^ $mask_right;
$ip_low = $ip & $mask_left;
$ip_high = $ip_low | $mask_right;
$range = [$ip_low, $ip_high];
return $range;
}
function rangeToSubnet(array $range)
{
$subnet = sprintf(
'%s/%d',
long2ip($range[0]),
32 - log($range[1] - $range[0] + 1, 2)
);
return $subnet;
}
Now that we're working with ranges represented by a lower and upper bound, the process of splitting a range involves creating two smaller ranges, one that covers the lower half of the original range, and the other that covers the upper half, e.g: if the original range was 8 to 15, the two new ranges would be 8 to 11, and 12 to 15.
Given this, we can implement a form of recursive binary search, by starting with the parent range, and then recursively splitting it into two halves if it overlaps with any of the used ranges, until we find either a completely unused range, or a completely used range:
function findUnusedSubnets($parent, array $used)
{
$parent = subnetToRange($parent);
$used = array_map('subnetToRange', $used);
$unused = array();
findUnusedRanges($parent, $used, $unused);
$unused = array_map('rangeToSubnet', $unused);
return $unused;
}
function findUnusedRanges($subnet, $used, &$unused)
{
foreach ($used as $range) {
if ($subnet[0] > $range[1] || $subnet[1] < $range[0]) {
continue; // subnet does not overlap used range
}
if ($subnet[0] >= $range[0] && $subnet[1] <= $range[1]) {
return; // subnet completely within used range
}
// split subnet into two smaller subnets and check again
$split = ($subnet[1] - $subnet[0] - 1) >> 1;
findUnusedRanges([$subnet[0], $subnet[0] + $split], $used, $unused);
findUnusedRanges([$subnet[1] - $split, $subnet[1]], $used, $unused);
return;
}
$unused[] = $subnet; // subnet does not overlap any used range
}
Combining these four functions will allow you to find unused subnets:
$parent = '10.0.0.0/24';
$used = [
'10.0.0.0/29',
'10.0.0.8/29',
'10.0.0.144/29'
];
var_dump(findUnusedSubnets($parent, $used));
Gives output (tested with php 5.6):
array(7) {
[0]=>
string(12) "10.0.0.16/28"
[1]=>
string(12) "10.0.0.32/27"
[2]=>
string(12) "10.0.0.64/26"
[3]=>
string(13) "10.0.0.128/28"
[4]=>
string(13) "10.0.0.152/29"
[5]=>
string(13) "10.0.0.160/27"
[6]=>
string(13) "10.0.0.192/26"
}