[Gllug] subnetting

[Mark Green]

-----Original Message-----
From: gllug-bounces at gllug.org.uk [mailto:gllug-bounces at gllug.org.uk] On
Behalf Of Ian Northeast
Sent: Monday, November 08, 2004 8:41 PM
To: Greater London Linux Users Group
Subject: Re: [Gllug] subnetting

Mark Green wrote:
>> The reason for this is because you are doing class based ip
subnetting,

>No he isn't. Classful subnetting does not permit /27. Using classful 
>subnetting, the only permissible netmask in a network number starting 
>200 is 255.255.255.0, /24. But, apart from being used to establish 
>defaults, classful subnetting should not have been enforced for the
last 
>9 years since CIDR came out.

> using class c, address so 200.100.99.31/27 is the broadcast address,
> using this subnet scheme will give you 8 subnets how ever only 6 of
them
> will be usable.

Would you care to explain this? Which 2 subnets would be unusable? Why?

Regards, Ian

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First off judging from what information was given I assume that Michaels
professor is introducing subnetting with classful subnetting first, yes
I realize that CIDR has changed things but classful subnetting is often
taught first, for example Cisco CCNA deals with classful subnetting
before moving on to classless subnetting. 

The IP address range being used is a class C, and so the number of bits
that will be borrowed from the host portion of the last octet will be 3
bits to give 8 subnets. From these 8 subnets the number of usable
subnets us equal to two to the power of the borrowed subnet bits then
minus two. The minus 2 is there for the reserved addresses of the
network ID and network broadcast.

(2 to power of 3) - 2 = 6 

Addition of the 3 bits that have been borrowed gives the subnet mask, in
this case the 3 left most bits so 128 + 64 + 32 = 224.

Subnet mask = 255.255.255.224
Slash format = /27

Subnetwork #        Subnetwork ID        Host Range       Broadcast ID

0                   200.100.99.0         .1-.30         200.100.99.31
1                   200.100.99.32        .33-.62        200.100.99.63
2                   200.100.99.64        .65-.94        200.100.99.95
3                   200.100.99.96        .97-.126       200.100.99.127

4                   200.100.99.128       .129-.158      200.100.99.159
5                   200.100.99.160       .161-.190      200.100.99.191
6                   200.100.99.192       .193-.222      200.100.99.223
7                   200.100.99.224       .225-.254      200.100.99.255

How ever both subnetwork 0 and 7 are unusable as they are reserved for
the network ID and broadcast.
  
The number of usable hosts on each subnet is equal to two to the power
of the bits remaining, minus two. The minus 2 is for the subnet ID and
subnet broadcast reserved addresses.

(2 to power of 5) - 2 = 30
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