Mailing List SIMS@mail.stalker.com Message #14775
From: Bill Cole <listbill@scconsult.com>
Subject: Re: (OT) Interpreting CIDR network info
Date: Wed, 29 Sep 2004 23:07:08 -0400
To: SIMS Discussions <SIMS@mail.stalker.com>
At 12:08 PM -0500 9/29/04, NetHead  imposed structure on a stream of electrons, yielding:
I realize this is a bit off-topic, but it is relevant in that sometimes
when trying to set up a blacklist for a range of IP's, my WHOIS reveals
only a CIDR designation for the network I am suspicious of.

For example, 218.175.0.0/16... how do I interpret that? I have read some
of the RFC's on CIDR notation, but it still boggles my mind. Would that
mean that they own the entire range from 218.175.0.0-218.175.254.254?

Actually through 218.175.255.255.

 There are a lot of large allocations out there, and Chunghwa/HINET's /16 you cite is actually part of a /12 (218.160.0.0-218.175.255.255) which still is nowhere near the largest contiguous block. Note Apple's 17.0.0.0/8 and a few dozen other /8 allocations in the low "class A" space.


Is there an easy way to read CIDR? or a tool that will do the conversion?

I've gotten really good at it by eye from extensive exposure, but I don't recommend that path. Basically, the dotted-quad dot boundaries are multiples of 8: /0 indicates all addresses, /8 is everything with the same first number, /16 is everything with the same first 2 numbers, and /24 is everything with the same first three numbers. From there you just use your memorized powers of 2 adjustments.... Like I said, I don't recommend that path. Note that using a non-boundary value as the base is perfectly legitimate, so 17.1.2.3/8 and 17.0.0.0/8 mean exactly the same thing.

There are lots of tools to do the conversion. Since Joe Laffey is an alumnus of this list and inhabitant of my former metropolitan home, I'll point you at http://www.laffeycomputer.com/whatmask.html


(I use the perl Net::CIDR and Net::CIDR::Lite modules, but I do so in one-liners as needed so I do not have a script handy... )
--
Bill Cole
bill@scconsult.com

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