Is the "Biggest subnet first" rule very critical

myb20

Hi

In my first CCNA skills test I was asked to divide a /24 subnet into /30 and /27 subnets. I proceeded as

x.x.x.0/30
x.x.x.32/27

My examiner gave 0 marks for it because I did not follow the biggest subnet first rule. According to him this should have been like

x.x.x.0/27
x.x.x.32/30

The question itself did not specify this rule but the examiner have given me reference of some examples in the course material where bigger subnets were created first. My understanding is that the order of subnets depend upon personal choice or suitability in a given environment. The bigger first could be a good practice but at least it is not that critical that one gets zero mark for a technically correct subnet. I did not find this rule anywhere on the Internet. Therefore, I am floating this question here. What is general opinion on this issue? Do I really deserve zero marks if the subnets were not in ascending order.

Thanks
Mirza

mochaaddict

You kind of answered your own question: " suitability in a given environment". If your instructor says its important and he/she is grading your work then..when in rome.

When I plan out a new site's addressing, I do address the requirement for larger subnets first and it is recommended. Of course one could work around it but why make your job more difficult then it needs to be.

my .02 cents.

ptilsen

We use larger subnets first as a good design practice that will generally provide better results in the future. There is no technical reason, to my knowledge, that the largest subnet IDs must be assigned first, but rather incontrovertible reasoning that it should always be assigned first.

Cisco Press wrote:

CCNA ICND2 Cert Guide: Variable Length Subnet Masks

Assigning the Largest Subnet IDs First

VLSM subnet assignment first occurs on paper, when the network engineer looks at a list of subnet IDs and chooses which subnet ID to use for which need in the network topology. For example, Figure 5-4 shows the need for two subnets with a /18 mask, three subnets with a /24 mask, and three subnets with a /30 mask. What specific subnets did the engineer choose? Which subnets could the engineer have chosen? This section explores how to answer these questions and how to go about choosing subnets.


When assigning subnets, follow this strategy: Choose the largest subnets first.

To show you why, we continue the example based in part on Figure 5-4. In that company, the LAN team will assign the subnets for the /18 and /24 subnets, and the WAN team will assign all the /30 subnets. The WAN team has already deployed some WAN links, and they have the political power and are unwilling to change. The WAN team has already used subnets 172.16.50.0/30, 172.16.100.0/30, 172.16.150.0/30, and 172.16.200.0/30.

Although the four WAN subnets have consumed a mere 16 addresses, unfortunately, those subnets have already busted the VLSM design. The four small subnet assignments have created an overlap with all four possible /18 subnets of network 172.16.0.0. Figure 5-5 shows the idea, with the four possible /18 subnets at the top and the overlapping WAN subnets at the bottom.


Figure 5-5 Overlaps Caused by Unfortunate Assignments of Smaller Subnets

When using mask /18, with Class B network 172.16.0.0, only four possible subnets exist: 172.16.0.0, 172.16.64.0, 172.16.128.0, and 172.16.192.0. The four small /30 WAN subnets each overlap with one of these four, as shown in Figure 5-5. How can you avoid making such mistakes? Either assign the smaller subnets from a much tighter range or assign the larger subnet IDs first, as suggested in this chapter. In this case, the LAN team could have allocated the first two /18 subnets first, and made the WAN team avoid using IP addresses from the first half of class B network 172.16.0.0.

Admittedly, the WAN team could not have been any more shortsighted in this contrived example. Regardless, it shows how a small subnet assignment can prevent you from having a larger subnet available. You should always strive to keep large holes open in your address space in anticipation of assigning large subnets in the future.

NetworkVeteran

myb20 wrote: »

The examiner have given me reference of some examples in the course material where bigger subnets were created first... Do I really deserve zero marks if the subnets were not in ascending order.

If as you say he taught this rule and provided several examples, yes, I would say it was fair to mark your answer incorrect. It's funny, as I read this, I was also thinking "When in Rome."

ptilsen

I think any instructor would mark the answer wrong, for the reasons I've given. In IT, especially in networking, if there is a clear best way to do something, that is the right way to do it, even if other methods still function. On CompTIA, Cisco, and Microsoft certification tests you will see questions that require you to select the "best" answer for this reason.

If the question were for some reason graded on a scale of correctness, it would not be "as wrong" as an answer that would not work (e.g., dividing the subnet into 15 /27 subnets, which wouldn't work mathematically), but still incorrect. In my experience on certification tests, answers are simply right or wrong, even if one answer is technically feasible or not as wrong as others.

myb20

Thank you guys. That is very much helpful. As I said this was my very first exam. Now I understand the logic of correctness. Very well explained. Many thanks.