[ih] The netmask

[Michael Grant]

------ Original Message ------
>From "Grant Taylor via Internet-history" 
<internet-history at elists.isoc.org>
To internet-history at elists.isoc.org
Date 08/01/2025 14:49:22
Subject Re: [ih] The netmask

>On 1/8/25 03:49, Michael Grant via Internet-history wrote:
>>Might this have been useful in cases where an internal network of machines was multi-homed on 2 separate networks?  As in, it had 2 separate prefixes but for simplicity the internal network and host numbering was the same and the internal routing was the masked bits? You wouldn't need this today with either NAT or a globally unique IP address and routing.
>
>Either I'm not understanding what you're suggesting or we're talking about two different things.
>
>The OS/390 bit was specifically about what we would consider to be the 24 contiguous bits for the /24 netmask.
>
>The 192.0.2.0/24 network is in the class B range, thus it has a network mask of 255.255.0.0.  However we want it to be a subset of that network (mas), thus a sub-net(mask) therein.
>
>11000000.00000000.00000010.00000000 192.0.2.0
>NNNNNNNN.NNNNNNNN.xxxxxxxx.xxxxxxxx network mask
>xxxxxxxx.xxxxxxxx.SSSSSSSS.xxxxxxxx sub-network mask
>xxxxxxxx.xxxxxxxx.xxxxxxxx.HHHHHHHH host bits
>NNNNNNNN.NNNNNNNN.SSSSSSSS.HHHHHHHH combination of network mask, sub-network mask, and host bits
>
>Conversely for 10.0.0.0/24
>
>00001010.00000000.00000000.00000000 10.0.0.0
>NNNNNNNN.xxxxxxxx.xxxxxxxx.xxxxxxxx network mask
>xxxxxxxx.SSSSSSSS.SSSSSSSS.xxxxxxxx sub-network mask
>xxxxxxxx.xxxxxxxx.xxxxxxxx.HHHHHHHH host bits
>NNNNNNNN.SSSSSSSS.SSSSSSSS.HHHHHHHH combination of network mask, sub-network mask, and host bits
>
>In OS/390 (et al.) the sub-net(work) mask is a separate configuration from the net(work) mask which is based on the class of the IP address.
>
>At least this is, and has been, my working understanding of OS/390's TCP/IP configuration on my P/390-E.
>
>The point that I'm trying to emphasize is that the sub-net(work) mask is a separate configuration value than the net(work) mask.  The net(work) mask and the sub-net(work) mask are combined to get what we now consider to be one aggregate thing and call sub-net / net-mask in common parlance.
>
>I suspect this harks back to an older version of the TCP/IP stack that only had support for the net(work) mask.  I speculate that newer versions of the software wanted to be compatible with configuration files for older versions and as such added the sub-net(work) portion as an additional parameter.
>
>But I've seen reference to the difference in net(work) mask and sub-net(work) mask in other things.
>
>We can also see some hints of this separation of net(work) mask ans bub-net(work) mask in Cisco IOS wherein `show ip route` groups multiple sub-et(work) prefixes under their net(work) prefix.  E.g.
>
>17.0.0.0/8 is variably subnetted ...
>    17.43.58.0/24 [200/0] via ...
>    ...
>170.187.0.0/16 is variably subnetted ...
>    170.187.6.0/23 [200/0] via  ...
>    ...
>216.152.65.0/24 [200/0] via ...
>136.237.0.0/24 is subnetted ...
>    136.237.45.0 [200/0] via ...
>    ...
>
>So the sub-net(work) being separate from the net(work) is not strictly isolated to OS/390.
>
>IMHO even the names suggest that they are, or at least were / used to be, separate things: "net(work) mask" and "sub-net(work) mask".
>
>The sub-net(work) mask wasn't anything like Cisco wildcards like I think you might be alluding to or what we might consider to be host-bits in various IPv6 network prefixes today.
>
>All of this is based on my observations and trying to understand them over the years and as such can be anywhere between not quite correct and completely wrong.
>
I see what you're saying.  In my thinking it's a little related but not 
the same thing.  And I may be mis-remembering something I misunderstood 
many years ago.