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The Supernetting / CIDR Chart

Posted in Supernetting & CIDR

supernetting-cidr-chart-1Because subnet masks can get very confusing, the creators of this wonderful network technology also made available a few things to make life somewhat easier.

The following Supernetting - CIDR chart is really a summary of what we've seen so far. It gives you a good idea of the networks we can combine and the result we'd see.


The Supernetting/CIDR chart

There are four columns available in our chart:

The CIDR Block, the Supernet Mask, Number of Class C Networks and the Number of Hosts column.

Class C
CIDR Block
Supernet Mask
Number of Class C Networks
Number of Hosts

Let's explain the meaning of each column, although you probably already know most of them.

The CIDR Block

The CIDR Block simply represents the number of bits used for the subnet mask. For example, /14 means 14 bits assigned to the subnet mask, it is a lot easier telling someone you have a 14 bit subnet mask rather than a subnet mask of :)

Note: In the above paragraph, we called the 14 bits as a subnet mask, when in fact it's a supernet mask, but because when you configure any network device, the field you will need to enter the value is usually named as the 'subnet mask', we decided to name it 'subnet mask' aswell, in order to avoid confusion.

We would also like you to pay particular attention to the CIDR Block /24, and /25 to /30. These blocks are highlighted in blue and yellow.

When we use a CIDR Block of 24 (24 bit subnet mask) we are not Supernetting ! This is a default subnet mask for a Class C network. With CIDR Blocks /25 to /30 we are actually Subnetting and not Supernetting !

Now you might wonder why we have them in the chart. The fact is that those particular CIDR Blocks are valid, regardless of whether applying them to a network means we are Subnetting and not Supernetting. If you have dealt with any ISPs and IP Address assignments, chances are you would have been given your IP Addresses in CIDR format.

A good example is if you wanted a permanent connection to your ISP and only required 2 IP Addresses, one for your router and one for your Firewall, you would be assigned one /30 CIDR Block. With such a subnet mask you will have 4 IP Addresses, from which 2 will be reserved (one for the Network address and one for the Broadcast address) and you're left with 2 that you can assign to your hosts (router and firewall).

The Supernet Mask

Basically, this is your Subnet mask. When you configure the devices that will be attached to the specified network, this is the value you will enter as a Subnet mask. It's also the decimal value the CIDR Block specifies. For example, a /24 CIDR block means a 24 bit Subnet mask, which in its turn translates to :) Simple stuff !

Number of Class C Networks

This number shows us how many Class C Networks are combined by using a specific Supernet mask or, if you like, CIDR Block. For example, the /24 CIDR Block, Supernet mask is 1 Class C Network, whereas a /20 CIDR Block, Supernet mask is 16 Class C networks.

Number Of Hosts

This value represents the number of hosts per Supernet. For example, when we use a /20 CIDR Block, which means a Subnet (or Supernet) mask of, we can have up to 4096 hosts. Pretty straightforward stuff.

There is one thing you must be careful of though ! The value 4096 does not represent the valid, usable IP Addresses. If you wanted to find out how many of these IP Addresses you can actually use, in other words, assign to hosts, then you simply take 2 IP Addresses from that number (the first and last IP Address), so you're left with 4094 IP Addresses to play with :)

Why take 2 away ? You shouldn't be asking questions like that if you have read the IP and Subnetting sections but I'll tell you anyway :) One is reserved for the Network Address and one for the Broadcast Address of that network !


That completes the explanation of the Supernetting/CIDR chart. You will see that Supernetting and Subnetting have quite a few things in common, and this is simply because they work on the same principle.

Again, if you have the whole topic, or certain sections hard to understand, you should give yourself a small break, and then come back for another round :)

Back to Supernetting & CIDR Section

Supernetting In-Depth Analysis - Learn to Identify and Create Supernets.

Posted in Supernetting & CIDR

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.

Understanding Supernetting - How Supernets Work

Posted in Supernetting & CIDR

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.

Enhanced Interior Gateway Routing Protocol - EIGRP

Posted in Routing Protocols


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.

Interior Gateway Protocol - IGRP

Posted in Routing Protocols


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.


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