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Say I have the following Network:

10.1.0.0/23

The first host of that network would be 10.10.0.1 and the last host of the network would be 10.1.1.254. The Network ID would be 10.1.0.0 and the Broadcast would be 10.1.1.255.

Now imagine I would subnet this network further into two /24 networks:

The first network would reach from 10.1.0.0-10.1.0.255 and the second network would reach from 10.1.1.0-10.1.1.255. So far so good. Yet, both of these /24 networks would still be part of that one larger /23 network.

Now, both of these networks are subnets from the larger, /23 network. However, there inside this network is now a host ID and a broadcast, and I learned that these cannot be assigned to hosts, even though they would obviously be valid host addresses for the /23 network:

(Network ID) = 10.1.1.0 (Broadcast) = 10.1.0.255

How are such matters resolved? Does that mean that once a network is subnetted, it's not advisable to assign hosts to the original network at all? Or am I having some kind of brainfart?

Take for example the following configuration of hosts on a local link:

Host 1: IP: 10.1.0.1 SNM: 255.255.254.0

Host 2: IP: 10.1.1.200 SNM: 255.255.255.0

From my understanding, host 1 would still be able to reach host 2 (f.e. via ping), because host 2 is still in the same subnet as host 1. However, host 2 wouldn't be able to ping host 1 because the logical AND operation on the SNM would tell him to send these packets to the default gateway. As there is no default gateway configured. the ping fails.

But if the configuration is like this:

Host 1: IP: 10.1.1.0 SNM: 255.255.254.0

Host 2: IP: 10.1.1.200 SNM: 255.255.255.0

Host 1 would have a valid IP config for his own /23 net, but since he got assigned the network-ID of the smaller /24 net, would he still be able to ping host 2?

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I think your confusion starts here:

Yet, both of these /24 networks would still be part of that one larger /23 network.

There is no longer a /23 network, because you have divided it into two /24s. You can summarize the two /24s as a /23, but the /23 subnet no longer exists as a subnet.

In other words, you don't have some hosts with a /23 mask, and others with a /24 mask. All your hosts will be on one or the other of the /24s.

You have also discovered a (small) drawback to subnetting: as you divide networks into smaller and smaller subnets, you lose space to network and broadcast addresses. If you take a /24 and divide it into /30's, you will lose half of the available address space.

  • One thing that the /23 network notation could be useful is for routing purposes: if both subnets are in the same area in the topology, routers far away would need only the /23 entry to route the packets, not the two /24 entries (it's called route summarization). – Peter Sep 1 '16 at 13:56

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