Mailing List Message #14780
From: Lewis Butler <>
Subject: Re: (OT) Interpreting CIDR network info
Date: Thu, 30 Sep 2004 13:29:54 -0600
To: SIMS Discussions <>
X-Mailer: Apple Mail (2.619)
On 29 Sep 2004, at 11:08, NetHead wrote:
For example, 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

Exactly right.

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

There are a bazillion pages on google that will calculate CIDRs and netmasks for you.  It is pretty easy to do yourself. is


/16 is

11111111.11111111.00000000.00000000 (first 16 bits are ON)

this means that anything in the address that is masked (has a 1 in the netmask) does not change, so the range is


if their address range was, say, /18 then the range would be:

11011010.10101111.00000000.00000000-11011010.10101111.00111111.11111111  (

if the address was a /24 then the range would be:

11011010.10101111.00000000.00000000-11011010.10101111.00000000.11111111 (

/8 is a Class A address, /16 is a Class B, and /24 is a Class C.  A Class C, of course, has 256 IPs in it.  A class B has 256^2 IPs (65536) and a class A has 256^3 IPs in it (16,777,216).  The entire IP address range contains 256^4 IPs (about 4.3 billion addresses).

"Let's get back to syntax of procmail and forget the syntax of fools." - Don

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