RE: CIDR Explanation

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To: "'ka55ad'" <ka55ad@gmail.com>, security-basics@securityfocus.com
Date: Tue, 21 Sep 2004 08:26:57 -0500

10.0.0.0/24

Means that the address starts with the first 8 bits (in this case, 10), and
then the next 24 bits are variable. So 10.0.0.0/24 is the same as saying
10.xxx.xxx.xxx.

10.1.0.0/16 -- The first 16 bits (10.1) stay the same, and the last 16 bits
change (10.1.xxx.xxx).

It's a little trickier mentally when the second part isn't a multiple of 8,
but just think of it always in terms of bits.

Hope that helps!

Ron Bowes
Information Protection Centre
Government Of Manitoba

-----Original Message-----
From: ka55ad [mailto:ka55ad@gmail.com]
Sent: Monday, September 20, 2004 8:39 AM
To: security-basics@securityfocus.com
Subject: CIDR Explanation

Hi List,

     I am hoping that somebody might be able to help me out or point
me in the right direction. For the life of me I can't seem to get a
good grasp on CIDR notation. I see a lot of CIDR addresses every day,
but I have trouble figuring out the IP addresses on the fly. I am not
a complete newbie - I am quite familiar with the OSI model, TCP
handshakes, etc but this one area stumps me.

     I am particularly interested in it because I am going to be
setting up a snort box soon as well as an IP chains firewall to
segregate parts of the network. I would much rather use CIDR since it
can be much quicker at times, but I don't want to use it right now due
to my lack of knowledge which can cause security issues. Can anybody
offer advice/help? Thanks.

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Computer Forensics Training at the InfoSec Institute. All of our class sizes
are guaranteed to be 12 students or less to facilitate one-on-one
interaction with one of our expert instructors. Gain the in-demand skills of
a certified computer examiner, learn to recover trace data left behind by
fraud, theft, and cybercrime perpetrators. Discover the source of computer
crime and abuse so that it never happens again.

http://www.infosecinstitute.com/courses/computer_forensics_training.html
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