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## IP Subnetting

I am facing some problem with the IP subnetting...

We have 3 classes being used for subnetting.

Class A: 1 to 126

1.0.0.0 - 126.255.255.255

Class B: 128 - 191

128.0.0.0 - 191.255.255.255

can we use network 191.255.0.0

Class C: 192 - 223

192.0.0.0 - 223.255.255.255

can we use network 223.255.255.0 ?

Another confusion is with regard to the number of networks available in a specific class.

We have last three octets reserved for Hosts and Subnets, which means we can have 16777214 hosts ... however what about the Number of Networks in Class A?

In few of the sites they have calculated

2^7-2 = 126 networks for class A.

Whereas in Cisco site they dont subtract 2 for determining Number of Networks ...

Plus why 2^7 for networks, why not 2^8?

Waiting for some quick responses

Thanks and regards

3 REPLIES
Cisco Employee

## Re: IP Subnetting

Hi,

Yes, the networks 191.255.0.0/16 and 223.255.255.0/24 can be used.

As to how many networks are available in a class A network:

It really depends on the masks you are using for subnetting. Today we are using Variable Length Subnet Mask (VLSM), which basically means that the length of the subnet mask does not have to be fixed to the same value everywhere in a network.

Lets take 10.0.0.0/8 as an example.

You can configure 16777216 host routes starting from 10.0.0.0/32, 10.0.0.2/32, ..., 10.255.255.254/32 to 10.255.255.255/32

OR 4194304 routes with /30 OR ... OR 2 routes with /9 namely 10.0.0.0/9 and 10.128.0.0/9

With VLSM you can also have combinations like 10.128.0.0/9 and 10.0.0.0/24 and 10.0.123.32/28 and 10.23.45.67/32 all in the same network.

As you might see now the answer to the question "What is the number of networks in a Class A?" can be between 2 and 16777216 and depends on the subnetmask(s) used.

The main task when subnetting is to avoid duplicate and overlapping IPs. You should not use e.g. 10.1.0.0/16 and 10.1.1.0/24 in two different places or have two hosts with e.g. 10.98.98.98 as then connectivity can be affected or prevented.

Hope this helps!

Regards,

Martin

New Member

## Re: IP Subnetting

Hi,

2^7 is because it start from 2^0 to 2^7 (0 to 7 is 8 bit) and (0 to 8 will be 9 bit) if we use 2^8

Class B: 128 - 191

128.0.0.0 - 191.255.255.255

can we use network 191.255.0.0 YES

Class C: 192 - 223

192.0.0.0 - 223.255.255.255

can we use network 223.255.255.0 YES

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml

Chao

Vishwa

New Member

## Re: IP Subnetting

Thanks for the help

2^7 concept is logical for sure

however when we talk abt determining the total number of hosts per network, then we use 8 instead of 7

e.g. Total Number of Hosts in Class C (using the default subnet mask of 255.255.255.0) is 2^8-2.

As per the above mentioned logic, should'nt we use 2^7-2 here?

thanks

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