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The Supernetting / CIDR Chart

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This articles build upon our previous pages covering  what is Supernetting (Route Summarization) & How Supernets Work and Supernetting In-Depth Analysis - Learn to Identify and Create Supernets.

The Supernet CIDR (Classless Inter-Domain Routing) chart serves as a reference tool that aids network administrators in understanding and implementing supernetting. It provides a systematic overview of supernetting and its associated CIDR notation, facilitating the calculation and identification of supernet addresses.

Below is our Supernetting/CIDR chart consisting of four columns: CIDR Block, Supernet Mask, Number of Class C Networks and the Number of Hosts:

Class C
CIDR Block
Supernet Mask
Number of Class C Networks
Number of Hosts
/14
255.252.0.0
1024
262144
/15
255.254.0.0
512
131072
/16
255.255.0.0
256
65536
/17
255.255.128.0
128
32768
/18
255.255.192.0
64
16384
/19
255.255.224.0
32
8192
/20
255.255.240.0
16
4096
/21
255.255.248.0
8
2048
/22
255.255.252.0
4
1024
/23
255.255.254.0
2
512
/24
255.255.255.0
1
254
/25
255.255.255.128
1/2
126
/26
255.255.255.192
1/4
62
/27
255.255.255.224
1/8
32
/28
255.255.255.240
1/16
16
/29
255.255.255.248
1/32
8
/30
255.255.255.252
1/64
4

Note: Green coloured entries represent supernetting, blue is the default Class C network and yellow entries represent subnetting.

Supernetting requires solid understanding of IP, subnetting and binary notation. Read through these sections if in doubt.

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.

We should also take note at the CIDR Block entries /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 because this is a default subnet mask for a Class C network. With CIDR Blocks /25 to /30 we are actually subnetting and not Supernetting.

CIDR blocks of /30 (4 IP addresses of which 2 are usable) are often assigned by ISPs to their customers and used to assigned their only two valid IP addresses to the equipment between the ISP and customer (usually routers or firewalls).

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 CIDR block of /24 means a 24bit subnet mask, which translates to 255.255.255.0.

To understand the coversion of binary bits to decimal numbers, read our article How Supernets Work.

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, 255.255.255.0 Supernet mask is 1 Class C Network, whereas a /20 CIDR Block, 255.255.240.0 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 255.255.240.0, we can have up to 4096 hosts.

Keep in mnd that the value of 4096 does not represent the number of valid (or usable) IP addresses. To calculate the number of valid IP addresses, you'll need to subtract two IPs (network & broadcast IP): 4096-2 =4094.

Summary

That completes our discussion on 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 :)

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