The previous article served as a good introduction to Supernetting (CIDR). We analyzed the Supernetting concept and how it helps bind multiple networks into one, decreasing the size of routing tables and saving valuable memory and CPU cycles on routers. This article take a step further by analyzing a Supernet example down to the bit level and covering best Supernet practices.

NOTE:This page requires you to have basic knowledge and understanding on Internet Protocol, Subnetting and Binary notation. These are covered in great detail on other pages and I recommend you have a quick look over these topics if you think you're not up to scratch.

Guideline - Rule to Supernetting / CIDR

Before we get in to deep waters, we must talk about the main rule that applies to creating Supernets. For our example, this rule dictates that, in order to create Supernets from Class C IP Addresses, the network address must be consecutive and the third octet of the first IP Address must be divisible by two.

If we had 8 networks we wanted to combine, then the third octet of the first IP address would need to be divisible by eight and not two.

There is one more rule you should know and this rule has to do with the routers of the network, which will need to work with the new changes. This rule dictates that all routers on the network must be running static routing or using a classless routing protocol such as RIP2 or OSPF. Classless routing protocols include the subnet mask information and can also pass supernetting information. Routing protocols such as RIP1 do NOT include subnet mask information and would just create problems!

The Example

Here is an example involving two companies that want to use Supernetting to solve their network requirements. We are going to determine which company mets the criteria for a Supernet (we are assuming the routers are setup in a way that will support supernetting):

As you can see, Company No.1's network passes the test, therefore we can Supernet its two networks.

This article explains basic concepts of Supernets - Supernetting and shows how Supernets work by combining multiple smaller class networks to create one large network, effectively replacing multiple routes with a single route.

Consider this realistic example: You work for a large ISP with a few hundred networks to which it provides services like Internet access, e-mail etc. These networks, which basically are your ISP's clients, consist of 254 host IPs each (One full Class C network for each client), and they all have a permanent connection to headquarters (represented by the yellow links) and from there the ISP has a direct connection to the Internet Backbone, as shown in the diagram below:

Our main focus are the two routers the ISP has, Router No.1 and Router No.2, because these will be affected when we supernet the smaller networks behind them.

Introduction

Enhanced Interior Gateway Routing Protocol (EIGRP) is another Cisco proprietary, hybrid (has feature of Distance Vector and Link State protocols), interior gateway protocol (IGP) used by routers to exchange routing information. EIGRP uses a composite metric composed of Bandwidth, Delay, Reliability, and Loading to determine the best path between two locations.

EIGRP can route IP, IPX and Appletalk. Along with IS-IS, it is one of the few multi-protocol routing protocols.

Introduction

Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary Distance-Vector routing protocol. This means that all your routers must be Cisco routers in order to use IGRP in your network, keep in mind that Windows 2000 now supports it as well because they have bought a licence from Cisco to use the protocol !

Cisco created this routing protocol to overcome the problems associated with RIP.

Introduction

Open Shortest Path First (OSPF) is a routing protocol developed for Internet Protocol (IP) networks by the interior gateway protocol (IGP) working group of the Internet Engineering Task Force (IETF). The working group was formed in 1988 to design an IGP based on the shortest path first (SPF) algorithm for use in the Internet. Similar to the Interior Gateway Routing Protocol (IGRP), OSPF was created because in the mid-1980s, the Routing Information Protocol (RIP) was increasingly unable to serve large, heterogeneous internetworks.

OSPF is a classless routing protocol, which means that in its updates, it includes the subnet of each route it knows about, thus, enabling variable-length subnet masks. With variable-length subnet masks, an IP network can be broken into many subnets of various sizes. This provides network administrators with extra network-configuration flexibility.These updates are multicasts at specific addresses (224.0.0.5 and 224.0.0.6).

For more information on OSPF visit our OSPF Routing Protocol section

The diagram below shows us the information that each field of an OSPF packet contains: