[H-GEN] MTA

Joel Michael joel at jmichael.bpa.nu
Sun Sep 7 04:40:44 EDT 2003


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irwa82 at froggy.com.au wrote:

>I don't understand why /24 works and /99 doesn't I though that /99 mean't
>that all ip address in the range of 192.168.100.0 to 192.168.100.99 would
>be accepted by the mail server and relayed.
>
>Obviously this is an incorrect understanding on my part, could somebody
>point me to documention or give a quick explanation of what /24 stuff is
>and why it works and /99 doesn't.
>
>  
>
Hi Anthony,

To understand "how it works", one needs to understand subnets, and the 
math behind them.  The quick answer is the /NN represents the number of 
bits in the network mask (with a maximum of 32 bits in IPv4).  This is 
called CIDR format, as opposed to the "old style" format e.g. 255.255.255.0

Getting back to your original question, you now mention that you want 
ranges in 192.168.100.0 through 192.168.100.99 to be able to relay.  You 
can not specify this in one single rule, as the upper edge does not fall 
on a network mask boundary.  You will need the following 3 networks to 
fill this rule:

 192.168.100.0/26
 192.168.100.64/27
 192.168.100.96/30

However, if you had chosen the upper edge to be .127, you would have 
only needed 1 rule: 192.168.100.0/25

See the 'netmask' program for a great tool to figure this kind of stuff 
out (http://packages.debian.org/netmask)

A good description of how subnets work is at 
http://www.scottharney.com/tcp-ip-class/x100.html and the following 3 pages.


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