This article is the second in a series of three which has been written for those who are, or may one day be responsible for designing an IP network.

Sit back, take a deep breath and prepare for another of my techie articles, this time on Super-netting, also known as Route Summarization or Route Aggregation

Introduction

The larger the network the bigger the routing table, in large internet-works, hundreds, or even thousands, of network addresses can exist, this will result in each Router taking longer to process information and route packets.

Super-netting is designed to improve convergence time, increase performance and reduce the size of the routing table and the number of routes that a router has by grouping multiple networks together and representing them as a single summarized network in the routing table.

NOTE: Super-netting can only be used with classless routing protocols, these extend the standard Class A, B, or C IP addressing scheme by using a subnet mask or mask length to indicate how routers must interpret an IP network ID. Classless routing protocols include the subnet mask along with the IP address when advertising routing information.

I will admit that I love IP Addressing - sad I know, but true! The best way I have found to explain supernetting to my students is to run through an example configuration

Example

An ISP has assigned a company eight network addresses which are outlined below:

• 192.168.168.0
• 192.168.169.0
• 192.168.170.0
• 192.168.171.0
• 192.168.172.0
• 192.168.173.0
• 192.168.174.0
• 192.168.175.0

Objective:
Your company MD wants to reduce the number of routing table entries to a single network ID. To acheive this the networks will have to be grouped together

How can you achieve this?
Review the first 8 bit group of numbers called an octet this will help you to determine the default network class and subnet mask.
As a reminder three classes of network are available:

• Class A Networks - The first octet of the IP Address is 1-126 and will use a subnet mask of 255.0.0.0

• Class B Networks - The first octet of the IP Address is 128-191 and will use a subnet mask of 255.255.0.0

• Class C Networks - The first octet of the IP Address is 192-223 and will use a subnet mask of 255.255.255.0

In this example the first octet is 192 which an example of a class C network with the default subnet mask of 255.255.255.0 shown as / 24 (as 24 bits are assigned as a network ID)

The first step to summarization is to convert the network ID’s into binary format.

The first, second and forth octets are identical for ALL of the networks therefore we only need to work on the third octet, this needs to be converted from decimal format into binary

To convert binary to decimal is reasonably straight forward each binary bit within each octet has a preset decimal value working from left to right of :

128, 64, 32, 16, 8, 4, 2 and 1

Now look for the column working from left to right with ALL the SAME digits (in this example ALL 1’s)

From this calculation the first 5 bits will represent the single network address giving a subnet mask for this octet of:
1 1 1 1 1 0 0 0

Which converted into decimal is:
128 + 64 + 32 + 16 + 8 = 248 or / 21

The super-net will be displayed using the first assigned network ID 192.168.168.0 with the new subnet mask of 255.255.248.0

This will be represented in the routing table as:

192.168.168.0 / 21

So thats it, you should now understand what a supernet is and how it is calculated - Until the next time - Good Luck