RE: CIDR Explanation

["David Gillett" <gillettdavid () fhda edu>]

  When you specify a subnet mask to an IP address, you're telling
<whoever> which of the 32 bits are network address, and which are
host address.  For instance, in the "Class C" mask 255.255.255.0,
the first 24 bits of address will be used as net/subnet address,
and the remaining 8 bits as host address.
  Since in 99.9999% of cases, the network/subnet bits are contiguous,
starting from the beginning of the address, CIDR notation writes
"the first 24 bits are net/subnet address bits" as "/24".

  In dotted notation, it's a bit easier to write and work with
masks that fall on octet boundaries.  In CIDR, it's no harder to
write "/23" (255.255.254.0) or "/25" (255.255.255.128) than it is
"/24".

David Gillett


> -----Original Message-----
> From: ka55ad [mailto:ka55ad () gmail com]
> Sent: Monday, September 20, 2004 6:39 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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