Security Basics mailing list archives
RE: CIDR Explanation
From: "Andrew Shore" <andrew.shore () holistecs com>
Date: Tue, 21 Sep 2004 11:29:38 +0100
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. ------------------------------------------------------------------------ --- 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 ------------------------------------------------------------------------ ---- --------------------------------------------------------------------------- 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 ----------------------------------------------------------------------------
Current thread:
- CIDR Explanation ka55ad (Sep 21)
- RE: CIDR Explanation David Gillett (Sep 21)
- Re: CIDR Explanation Bob Radvanovsky (Sep 21)
- Re: CIDR Explanation Alexandros Papadopoulos (Sep 23)
- <Possible follow-ups>
- RE: CIDR Explanation BĂ©noni MARTIN (Sep 21)
- RE: CIDR Explanation Andrew Shore (Sep 21)
- RE: CIDR Explanation Bowes, Ronald (EST) (Sep 21)
- RE: CIDR Explanation David Gillett (Sep 21)
- CIDR Explanation - A good web site to teach you. Chad Thomsen (Sep 23)
- RE: CIDR Explanation David Gillett (Sep 21)
- Re: CIDR Explanation Ed Spencer (Sep 21)
- RE: CIDR Explanation Mike (Sep 21)
- Re: CIDR Explanation Travis Schack (Sep 21)
- RE: CIDR Explanation Bowes, Ronald (EST) (Sep 22)
- Re: CIDR Explanation ka55ad (Sep 22)