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Q: Five-octet network ID's ( Answered,   1 Comment )
Subject: Five-octet network ID's
Category: Computers > Internet
Asked by: idunnoman-ga
List Price: $15.00
Posted: 12 Nov 2006 09:21 PST
Expires: 12 Dec 2006 09:21 PST
Question ID: 782103
I am studying for my MCSE and have noticed in the study materials that
it mentions network ID's containing five subnets. (e.g.  I am not familiar with this convention and don't
know what it's used for.  I am used to regular old IP addresses with
four subnets.  An explanation would be great.

Request for Question Clarification by sublime1-ga on 12 Nov 2006 12:31 PST

I'm wondering if you're seeing the word 'subnet' and understanding
it to mean a fifth decimal section (octet) in the IP address, or
do the study materials actually contain an address such as the 
example you've given (

If no such example was given, I think you're misunderstanding the
word subnet. The 4 sections of the IP address are called 'octets',
from their derivation from binary math.

'Subnets' refer to a portion of an IP network defined by a subnet
mask. Devices on the same subnet have the same subnet mask.

An informative discussion of all of this is provided in this question
posted by bagelboy and the answers given on the MCSE forum on the
VelocityReviews site:

"For a class C network, a subnet mask of
 (binary 11111111 11111111 11111111 11100000) indicates that
 the first three octets and the first 3 bits [digits] of the
 last octet identify the subnetted network ID. The remaining
 5 bits identify the hosts on that subnet. Because 3 bits are
 used to define the subnetting structure of this class C network,
 the network will support up to 8 subnets and up to 30 hosts per
More on the page:

Let me know if this satisfies your interests...

Subject: Re: Five-octet network ID's
Answered By: keystroke-ga on 12 Nov 2006 18:16 PST
Hello idunnoman,

Thank you for your question.
I feel absolutely certain that I know exactly what you are referring
to. You are talking about the short hand notation of Network + host
address with subnet identifiers.
For example:
means the subnet mask of
This gives you one subnet on the network with a network address of and a broadcast address of
The notation shown as
means your subnet mask is shown as
This gives you a network address of and a broadcast
address of, also a network address of
with a broadcast address of, also a network address of and a broadcast address of
Remember that Microsoft does not accept subnet zero! Things get a lot
hairier when you get into supernets. I will let you find that out for
The final number in that 5 octet address just defines the number of
bits used for the network MASK of the IP ADDRESS that is borrowed from
the host identifier and from there you can work out the bits used for
the host identifiers.
I have to admit that I got a little stuck with this but I kept with it
and it became clear at the end. What books are you reading? I would
really advise you to read the MSPRESS 70-291 and 70-293 books
dedicated to the MCSE syllabus. They are by far the best books I read
pertaining to the MCSE and you will not go wrong with them.

Let me know if you need any additional clarification and I'll be glad
to assist you.

Clarification of Answer by keystroke-ga on 12 Nov 2006 18:18 PST
This is also known as Classless InterDomain Routing or CIDR. This
webpage will really help you out with the understanding.
Wikipedia Entry-- Classless Inter-Domain Routing
Subject: Re: Five-octet network ID's
From: ubiquity-ga on 14 Nov 2006 08:53 PST
Maybe he means the MAC address

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