subnetting, nested subnetting, VLSM & broadcast addresses

Question

A teacher told us about VLSM and "nested subnetting":

• you should NOT use all-1 / all-0 subnets
• the same public network you got, can be split up in subnetworks of different size / having different subnet masks
• you can nest subnets, so that the same IP actually belongs to 2 subnetworks: in the example below the range of B is from 193.174.26.161 to 190, but C is nested inside from 193.174.26.189 to 193.174.26.190; furthermore B & C are nested in A
• the aim of this is to waste less IPs
• it is not supported by routers, but theoretically it would work

The teacher gave us examples with requirements like these:

• you got 193.174.26.0 /24 from your ISP
• A: 140 hosts
• B: 20 hosts
• C: 2 hosts
• given: C should be nested in B

--> using equally sized subnets without nesting will not work

The solution:

┌───────┬──────────────┬──────────┬──────┬──────────┬─────────┬─────────┐
│network│net address   │last octet│suffix│first host│last host│broadcast│
├───────┼──────────────┼──────────┼──────┼──────────┼─────────┼─────────┤
│A      │193.174.26.0  │0000 0000 │/24   │1         │254      │255      │
├───────┼──────────────┼──────────┼──────┼──────────┼─────────┼─────────┤
│B      │193.174.26.160│1010 0000 │/27   │161       │190      │191      │
├───────┼──────────────┼──────────┼──────┼──────────┼─────────┼─────────┤
│C      │193.174.26.188│1011 1100 │/30   │189       │190      │191      │
└───────┴──────────────┴──────────┴──────┴──────────┴─────────┴─────────┘

I had never head about it and could not find very much information, so I have some questions about it:

1. I think C is an all-1-subnet, am I right?
2. What happens if a router (e.g. the router connecting the local network to the internet) receives a packet with target = 193.174.26.255? How does it decide, whether to route it to all hosts (everyone between 193.174.26.1 and 193.174.26.254, no matter, whether it is in subnet B or C) or just to the hosts, that are directly in network 193.174.26.0 (the root of the nested networks)
3. Who will receive packets addressed to 193.174.26.191? All hosts in B including C or only the hosts in C?
4. In general, even if you do not use VLSM and nesting, but you ALLOW all-0 / all-1-networks: How do you distinguish between the network / broadcast address of the "super-network" and the sub-network? (following the linked article it should work meanwhile)
5. Does anyone use this technique in real life?

Cisco article about all-1 and all-0 subnets http://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/13711-40.html

Answer 1

"Nesting" (overlapping networks) requires proxy-arp and therefore SHOULD be avoided at all costs. No enterprise router will allow such a broken configuration -- each interface/subnet must be completely independent, which means out in the real world, where real IP addresses are routed, this method of "conservation" cannot be used. (aka: nonsense) ^[*]

It SHOULD not be attempted by anyone not thoroughly versed in networking. (i.e. if you haven't been designing, setting up, and maintaining large, complex networks for a decade or more, you shouldn't even be talking about this type of damage.)

(Full disclosure)
I'm doing this exact thing in an OpenStack development environment right now. 192.168.xx.0/24 has a /29 behind one of the machines in the larger /24. That machine has to have a number of specific, non-default setting changed to pretend to be hosts within the /29 slice. (aka proxy-arp) Yes, I can add a route for the /29 on the router, but the machines in the /24 still won't be able to talk to the /29 because their larger netmask means they're link-local; I'd have to add that /29 route to all the machines in the /24 for them to work.

All-0 and All-1
Those concepts haven't had any tangible meaning in modern networking for decades. Nothing you're likely to run into on the internet makes any assumptions about network size -- everything is classless now. Yes, there used to be issues using an all-0 (or 1) subnet -- say 199.72.0.0/24 (the first subnet from 199.72.0.0/16) ^{(true story)} -- because some random system on the internet (AIX) applied class logic to the range. Nothing does that today. So, with 199.72.0.0/16, the address range is 0.0 to 255.255 -- with the those too addresses being the /16's network and broadcast addresses. Those are always the /16's network and broadcast, even if a /24 were nested with it somewhere.

The active netmask ALWAYS defines the network and broadcast. Yes, that means a nested construct has multiple broadcast addresses, but due to different netmasks, nodes within different zones (sub-network, parent-network, ...) listen to different addresses. At layer-2 (ethernet), all hosts in the same domain (eg. vlan) see the same broadcasts but the host will filter out, at layer-3, the "foreign" broadcasts, unless they're sent to the "all nodes" broadcast address of 255.255.255.255.

^{[*] ISPs wanting to conserve space like this do it via bridging, but that has it's own problems.
[*] I warned my idiot ("we know more than you") coworkers not to use 199.72.0.0/24, but they did it anyway -- putting the webdev desktops in 0.0/25. A day later came the "What. Did. I. Tell. You." after complaints from every single person about random places on the internet they simply couldn't get. That was in 1997.}

Answer 2

Your teacher, unfortunately, has made an error. You can't "nest" or overlap subnets. In fact, many routers will not let you configure addresses this way -- they will give you an error because they don't know which interface to use for a given "nested" address.

All your questions are good ones -- they point out the problems with trying to overlap subnets and why that won't work.

I can say that the answer to 5 is No.