# Netmatics Supernet Calculator

Netmatics Supernet Calculator can analyze a set of addresses and calculate supernets. Supernetting, also known as route summarization or aggregation, is required in large networks to keep routing table size manageable and latency low. It is one of the key goals of design and planning for network scalability. The calculator is easy to use and this tutorial shows its features.

As the name suggests ‘supernetting’ is the reverse of ‘subnetting’ that divides a large network into smaller networks or subnets. In supernetting smaller networks in routing table are aggregated into a larger network so fewer routes have to be advertised and maintained in remote routers. The technique requires routing protocol to support CIDR and variable length masks.

Let’s look at a simple example to explain the concept. Suppose a routing table shows following 4 subnets connected to interfaces of a router:
200.16.0.0/24
200.16.1.0/24
200.16.2.0/24
200.16.3.0/24

These subnets can be represented by one address 200.16.0.0/22, which is the supernet in this case, and meets the following two conditions:

1. All addresses in any one of the four subnets, with prefix /24, are in 200.16.0.0/22.
2. All addresses in 200.16.0.0/22 will be in one of the four subnets.

With this information instead of advertising routes to the four subnets individually the router can just advertise 200.16.0.0/22 and the remote routers store just the supernet address instead of routes to all four subnet.

The Netmatics Supernet Calculator calculates supernets that meet the two conditions above for a given set of subnet addresses.   For those interested to learn how to manually calculate supernets there is a good blog entry at Subnetting Made Easy blog by Chris Bloomfield.

Now lets look at 3 examples of supernetting using the calculator.

Example 1: Let’s start with a simple example where the addresses neatly fit in a Supernet. Find the supernet for the following 8 subnets
204.32.16.0/24
204.32.17.0/24
204.32.18.0/24
204.32.19.0/24
204.32.20.0/24
204.32.21.0/24
204.32.22.0/24
204.32.23.0/24

Using the tool is very simple – just copy or enter the list of addresses to be aggregated in the input box, click the aggregate button and we get the route summary, as shown in figure 1:

Figure 1

The Route Summary outputs the supernet addresses, in bold.  You can click the icon beside to expand or collapse the subnet addresses contained in the supernet address.

Example 2: We will now look at an example where the addresses that we want to aggregate do not fit in one supernet address – often the case in real life so lets see what to expect from the tool.

172.17.0.0/16
172.18.0.0/16
172.19.0.0/16
172.20.0.0/16
172.21.0.0/16
172.22.0.0/16
172.23.0.0/16

Figure 2

As shown in the Route Summarization Table

• 172.17.0.0/16 does not fit and must remain as is.
• 172.18.0.0/15 is the supernet of 172.18.0.0/16 and 172.19.0.0/16
• 172.20.0.0/14 is the supernet of 172.20.0.0/16 to 172.23.0.0/16

The tool has summarized the 7 routes into 3.

Example 3: Lets consider one more example where subnet addresses have different prefix. Find the supernet of

204.15.18.0/23
204.15.16.0/24
204.15.17.0/24

Simply enter these addresses in the input box and click the ‘Aggregate’ button.

Following screen shot shows the input and output.

Figure 3

The calculated supernet is 204.15.16.0/22 because the tool is able to detect that 204.15.18.0/23 is the supernet of 204.15.18.0/24 and 204.15.19.0/24, which have the same prefix as the other two subnets.

Give it a test drive – you may find it useful.