ITEM: E9981L

Please explain impact of applying a partial subnet mask

QUESTION:

Our IP address is 132.147.160.5.  We have been running a subnet mask of
255.255.0.0, but this was inadvertently changed to 255.255.240.0.  What's
the impact of this and how does subnet masking work?

RESPONSE:

Your IP address is a Class B (because the first number is between 128 and
191) so, by default, the first two full octets identify the network part of
the address.  The remaining two octets identify the host number on the 
network.  In your case, your network is 132.147 and the host part is 160.5.

Applying a subnet mask alters the interpretation of the network and host
parts.  If you were to apply a subnet of 255.255.255.0, the network would
be 132.147.160 and the host part is 5.  

In your case the subnet was 255.255.240.0.  This mean that the third octet
is split -- the first four bits are for the network part and the last four
are for the host part.  You can see this from the ifconfig tr0 command:

   inet 132.147.160.5   netmask 0xfffff000   broadcast 132.147.175.255
                                     /
         hex equiv of 255.255.240.0 /

Notice that you are on ip network 132.147.160.0 -- this would be true for
all addresses with a third octet of 160 through 175.  The broadcast for all
of these addresses is 132.147.255 -- notice that the third octet is different
with the broadcast.

As you can see, you really have to understand subnet addressing to realize the
effect of subnetting on a partial octet.  It is for this reason that I recommend
subnetting on full octets if at all possible.  Subnetting on three octets gives
a possibility of many different networks but few hosts.  Subnetting on one octet
gives a few networks, but many hosts per network.



Support Line: Please explain impact of applying a partial subnet mask ITEM: E9981L

Dated: November 1993 Category: N/A

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