RE: CIDR Explanation

["Mike" <mike () superiorholidayadventures ca>]

The way I understand it, is that it's an easier way of writing subnets.

Normally you'd write 192.168.0.1 as the IP and 255.255.255.0 as the
netmask.  With CIDR you can just write 192.168.0.1/24.  The 24
represents how many bits in the netmask are 1's.

Each set of numbers in the netmask is 8 bits, so in this case
255.255.255.0 there are 24 1's (11111111.11111111.11111111.00000000).
If you were to write 192.168.0.1/12 the netmask would be 255.240.0.0 -
twelve 1's (11111111.11110000.00000000.00000000).

Hope that helps,

Mike Fetherston

> -----Original Message-----
> From: ka55ad [mailto:ka55ad () gmail com]
> Sent: Monday, September 20, 2004 9:39 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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