RE: CIDR Explanation

["Andrew Shore" <andrew.shore () holistecs com>]

A subnet mask simply says how many bits of the address are network and
how many are host.

Eg 10.2.3.55 255.255.255.0 means

The network is 10.2.3.0 and this host is 55 on that network

CIDR is just a way to save typing :)

In the above example the mask is 255.255.255.0 each number represents on
octet of a 32 bit number (in binary 11111111.11111111.11111111.00000000
3x8 1's and 8 0's) There are 24 1's so 24 bit mask or /24 in CIDR
notation. We use the doted notation because its easier to remember.


If you sub net the above example into  two networks (allowing subnet
zero) the mask would become 255.255.255.128 in (binary
11111111.11111111.11111111.10000000) which give a /25 CIDR network.

Basically count the 1's in the mask.

Of course this does imply that you have a good grasp of subnetting.

Check out Todd Lamels CCNA book for some good lessons on subnetting

-----Original Message-----
From: ka55ad [mailto:ka55ad () gmail com] 
Sent: 20 September 2004 14:39
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.

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