RE: CIDR Explanation

From: Mike (mike_at_superiorholidayadventures.ca)
Date: 09/21/04

  • Next message: Bob Radvanovsky: "Re: CIDR Explanation" 
    Date: Tue, 21 Sep 2004 08:24:12 -0400
To: "ka55ad" <ka55ad@gmail.com>, <security-basics@securityfocus.com>

    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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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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