Netmatics IP Subnet Planner

Netmatics subnetting tool splits an IPv4 address space into subnets of fixed or variable mask length. The tool generates a table of subnet addresses based on user provided addressing requirements.

This tutorial shows how to use the netmatics subnetting tool and it is assumed that the reader is familiar with IPv4 addressing concepts such as Classless Inter-Domain Routing (CIDR), Variable Length Subnet Mask (VLSM) etc. IP Addressing and Subnetting For New Users is a good introductory guide to understand fundamental concepts of IPv4 addressing.

The subnet tool requires information about the address space from which the subnets are to be created and the type of subnetting to be done, for example if the subnets should have fixed or variable length mask. The tool then

  • Calculates optimal number of subnets and hosts in each subnet
  • Divides the address space in subnet
  • Produces a results table that shows address range for each subnet created

The best approach is to see the tool in action with examples and for that we will use a simple topology shown below with the number of hosts required in each subnet using the address space

Figure 1: Example topology with subnets and number of hosts required.

In this example, 5 subnet have to be created and each subnet requires a minimum number of host addresses – net A requires 14 hosts, net B requires 28 hosts, net C require 2 hosts and so on.  There are two ways we can create the subnet masks Fixed and Variable Length Subnet Masks, FLSM and VLSM, respectively.

Example 1 – Fixed Length Subnet Mask (FLSM): In FLSM all subnets have same number of prefix bits, which means all subnets will have same number of available hosts.   We have to provide the tool address space, IP address and mask, to be subnetted  and either minimum number of subnets to be created or minimum number of hosts per subnet.

In our example, there are 5 subnets for which we have to create the address plan.  Enter the address space with mask then choose “Fixed” for “Masked Length”.  Choose “Number of Subnets” and enter the number of subnets to be created, 5, as shown below:

Figure 2: Fixed Length Mask Subnetting with Number of Subnets

Alternatively, for fixed length mask subnets, we could have provided the minimum number of hosts per subnet.  Since all subnets will have equal number of hosts, we use the hosts required by the largest subnet.  In our example, that is 28 hosts required in ‘net B’.  Following screenshot shows the ‘Hosts per subnet’ as inputs

Figure 3: Fixed Length Subnet Mask Input with Number of Hosts

Clicking the ‘Submit’ button produces the subnet table with 2 main outputs displaying information about the address space as well as information about each subnet

Figure 4: Fixed Length Subnet Mask Plan

The tool allocated 8 subnets (smallest power of 2 larger than 5, the required number subnets) and gave 30 host addresses which would accommodate the largest subnet required to have 28 hosts.

Example 2 – Variable Length Subnet Mask: To calculate subnet address plan with variable length mask choose ‘Variable’ mask length and enter subnet name and number of hosts separated by comma, as shown below:

Figure 5: VLSM plan input

That’s it – click ‘Submit’ and the tool will calculate the output table shown below:

Figure 6: VLSM Address Plan

The tool calculates optimal number of host addresses per subnet that meets the subnet requirement, for example both subnets ‘net A’ and ‘net D’ were allocated 14 hosts, which meet or exceed the number of host addresses required in ‘net A’ and ‘net D’.  As expected, VLSM is much more efficient in utilizing the address space with 154 addresses still available for allocation.  Also note the tool allocates addresses in descending order of the number of hosts required. This allows efficient address utilization and reduces wastage.

About Furrukh Fahim
A software engineer and a carpenter interested in making useful things, be it a software solution or a pergola.

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