Stupid Question maybe?

Wed Dec 19 15:24:08 UTC 2018

On Wed, 2018-12-19 at 14:54 +0000, Naslund, Steve wrote:
> I am wondering how a netmask could be not contiguous when the network
> portion of the address must be contiguous.  I suppose a bit mask
> could certainly be anything you want but a netmask specifically
> identifies the network portion of an address.

Before CIDR, subnets allowed further subdividing Classful addresses and
the mask bits after the network part could be non-contiguous. For a
class A network the first 8 bits covered the classful network, but the
remaining subnet bits did not have to be contiguous. Most network
admins made the subnet part contiguous, but allowing non-contiguous
subnet masks simplified the actual implementation. There was no need to
check if the bits were contiguous in the code.

Also subnet masks had to be the exactly the same on all devices.  You
could not have variable length masks.  A common practice if you could
get a Class B network (16 bit network part) was to use a 24 bit mask to
divide the network into 254 subnetworks which was adequate for most
purposes at the time.
-- 
Smoot Carl-Mitchell
System/Network Architect
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