VLSM: to each his own?

mguy

In VLSM design is there a rule of starting the subnet from biggest network starting at 0? Or does it not matter how you distribute the IP addresses just as long as you follow the subnet rules?

Say I have four disparate networks I need to create from 192.168.1.0/24

Network A: 62 users
Network B: 54 users
Network C: 14 users
Network 6 users

I would start it at the highest then lowest.

Network A: 192.168.1.0/26 (.1 - .62)
Network B: 192.168.1.64/26 (.65 - .126)
Network C: 192.168.1.128/28 (.129 - .142)
Network 192.168.1.144/29 (.145 - .150)

Is this a good way to do it? I can also do this:

Network 192.168.1.56/29 (.57 - .62)
Network A: 192.168.1.64/26 (.65 - .126)
Network B: 192.168.1.128/26 (.129 - .162)
Network C: 192.168.1.162/28 (.163 - .176)

or this

Network 192.168.1.0/29 (.1 - .6)
Network A: 192.168.1.64/26 (.65 - .126)
Network B: 192.168.1.128/26 (.129 - .162)
Network C: 192.168.1.162/28 (.163 - .176)

So does it matter or is it to each his own?

oli356

As far as I remember you should do it the way you did it first, with the most users first..

m3zilla

Technically, you can do it however you want. But with the 2nd option, what do you plan on doing with 192.168.1.8 - 192.168.1.64?

ptilsen

It's not simply a matter of preference. Always do larger subnets first, as a best practice and to prevent future issues. Below is a copy/pasted quote from another thread.

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.

fadhil

whatever you do is write but start subnetting a subnetwork with higher number of user