[ih] The netmask

[Michael Grant]

>From "Grant Taylor via Internet-history" 
<internet-history at elists.isoc.org>
>I've run into extremely obtuse IP configurations on things designed very early on in CIDR's lifetime wherein the sub-net is the number of bits /after/ the *netmask*.  E.g. 192.0.2.0/24 (Tnet-Net-1) has a netmask of 255.255.0.0 and a sub-netmask of 0.0.255.0.
>
>Yes, I am thinking about OS/390's TCP/IP configuration.
>
>In retrospect, I've seen hints of this in other systems too.  I think that some Cisco IOS output hints at this if you know how to squint at it to see the pattern.
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.