Supernetting After Network is Built?

[Israel Logan]

I'm in one of those typical situations where my organization is given 3 networks (x.x.12.x, .13.x, .14.x)  but we're running out of ip addresses for clients.  So I'm studying supernets and sort of going, "aha!".  But I'm not sure of the practical application or consequences of supernetting after the network is already built.

First and foremost, is anyone aware of legal limitations of implementing supernetting?  I mean, if there is an organization I go through to get three /24 networks for 252 hosts and I later decide that I need 510 ip addresses... can I just go and change my prefix to a /20 in order to supernet my x.x.12,.13,.14 networks and have the additional IP spaces? 

Second, if I were to go in and implement supernetting of the existing IP space that I was given, I'd have to change that anyplace the /24 prefix was, right?  I mean... that could break all sorts of things rather it be on the systems or networks side, right?  Maybe I'm overreacting on this angle of supernetting.  I mean, I can imagine adding new VLANs and just having them within the newly created supernets.  So for example if I used to have a .13.x subnetted VLAN that was x.x.13.0 /24 mask of 255.255.255.0 I could now make a a new VLAN for additional hosts of x.x.13.0 /20 mask of 255.255.240.0 which would give me many more addresses to work with.

Okay... just labbed it up and yeah...don't work so pretty.  So basically, doing something like supernetting would require a complete overhauls of any previously programmed VLANs.  What a mess!

I mean... we have dozens of VLANs.

I don't see supernetting working practially in my situations.

Thanks for any help. 

[Reza Sharifi · Accepted solution]

So, for your first question, there is no legal limitation or any issues with you making changes in the IP segments you have. Assuming your segments are public IPs, the only thing you would need to do is to talk to your providers, so they can advertise your segments correctly.  For example, if you originally told the provider to advertise a /23 for 2 /24s and now you want to advertise just 2 24/s because you are segmenting your network, you do want to tell them to advertise 2 /24s for you.

As for the second question, you are correct.  If you want to make changes to your subnets and the network is already built, it will require outages to make the changes. It can get messy very quickly if you don't do your home work ahead of time with good planning and documentations.

HTH

[Israel Logan]

Reza, 

Thank you for topic 2.  Knowing that will probably mean we won't update the IP spaces until the next major update.

For topic 1, thanks for that too but don't you mean they would advertise /20?  For a x.x.12.x, .13.x, .14.x, group of previously /24 255.255.255.0 networks that are now 255.255.240.0 supernetted/aggregated wouldn't that require a /20 advertisement for the newly supernetted subnet?

So the new 'supernet' would be x.x.12.x /20?

[Reza Sharifi]

Hi Israel,

I was just providing an example.  For your subnets, yes if you have 4 /24s than the provider can advertise each individually or one /22. A /21 will cover 8 /24 subnets and a /20 will cover 16 /24 subnets. Usually most smaller organizations are ok with advertising just a summary route i.e /23, 22, etc..

HTH 

[Israel Logan]

Reza, thanks again.  Again though...My situation would require /20 since the first four bits are used and differ when making a x.x.12.x, .13.x and .14.x.  

I think we're saying nearly the same thing but just wanted to clarify the slight difference for my network.

Based on this https://learningnetwork.cisco.com/blogs/journey-back-to-ccna/2012/05/31/supernetting-review 

Edit:

But Reza... as you point out the /20 would be for 16 networks so wouldn't that rob potential networks that could have been used?  I think this would be another point of contention with the people who give out the IP space.

But then again, if we would have just done a /22 from the start we could have had 200 more IP spaces than we have right now (with 3 /24's) even though it would have been at the expense of more networks available to other users.  Or...we could have just done two /25's... hmmm this is getting interesting

[Reza Sharifi]

Hi Israel,

Yes, so if you have 3 /24s than you can advertise one /23 and one 24 or just 3 /24s.  You can't advertise a /22, because you don't own the 4th subnet, which in your case is 15.x.

Most likely, the provider has assigned 15.x to a different organization.

HTH