RE: CIDR Explanation

From: Andrew Shore (andrew.shore_at_holistecs.com)
Date: 09/21/04

  • Next message: Lonely Wolf: "[RE] CIDR Explanation" 
    Date: Tue, 21 Sep 2004 11:29:38 +0100
To: "ka55ad" <ka55ad@gmail.com>, <security-basics@securityfocus.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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  • Next message: Lonely Wolf: "[RE] CIDR Explanation"

    Relevant Pages

    • Re: CIDR Explanation
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