Feature request: IPNetwork Subtract

Looking at the source code, I see a method called: TrySubstractNetwork. I attempted to use a slightly modified version of this function and it was extremely slow.

I am trying to take two lists IncludedRanages and ExcludedRanges and have something consolidate adjacent and overlapping ranges/networks. Is this something easily added?

[lduchosal]

@RobThree thanks, I’ll dig into your proposal.

[RobThree]

Rather than trying to subtract ip address via range, I found that adding subnet via binary elimination might be faster. The idea is to split network into two until all the split networks are free of the subtract target. These subnets can be farther split to subtract multiple networks. Finally the split parts free from subtract networks can be joined together.

Exactly. This is how it’s supposed to be done and can be very fast since all you do is split recursively. I have implemented the algorithm here (disclaimer, stating the obvious: author here).

For an example:

var network = NetworkHelper.Parse("192.168.0.0/16");
var desired = NetworkHelper.Parse("192.168.10.16/28");
var result = network.Extract(desired);

// Result:
// 192.168.0.0/21
// 192.168.8.0/23
// 192.168.10.0/28
// 192.168.10.16/28
// 192.168.10.32/27
// 192.168.10.64/26
// 192.168.10.128/25
// 192.168.11.0/24
// 192.168.12.0/22
// 192.168.16.0/20
// 192.168.32.0/19
// 192.168.64.0/18
// 192.168.128.0/17

You can also pass “0.0.0.0/28” to ‘extract’ any /28 from the given network. Works with IPv6 as well. What may help is using a visual subnet calculator like this one (no affiliation) to help visualize the concept. @lduchosal’s exampe would be done as:

var network = NetworkHelper.Parse("0.0.0.0/0");
var desired = NetworkHelper.Parse("10.0.0.1/32");

var result = network.Extract(desired);
var resulttext = string.Join("\n", result.Select(n => $"{n.Prefix}/{n.PrefixLength}"));