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My question is quite simple: let us say there is a 10.0.0.0/8 with the subnet mask of 255.0.0.0 (first 8 bits used up for network address).

Let us say, one address in our subnet is 10.0.0.1 (subnet mask /8)

Now, we decide to create a subnet further, and use 3 bits from the remaining 24 to form its network ID, thus creating a /11 network.

To start with, we represent the first subnet by keeping all the 3 bits as 0, thus forming a 10.0.0.0/11 network.

In that subnetwork too, we create an IP address: 10.0.0.1 (with a subnet mask of /11)

Thus we have two host address that are same, albeit with the different network mask. My question is, does there exist any system to prevent this? Because from what I have studied about subnets, this is indeed possible theoretically.

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    i don't think there is a general option. this is an erroneus configuration that results in an incorrectly working network. depending on how you actually separate subnets, there may be an option somewhere to detect that there is an overlap in configuration or the option to detect that something is not working.
    – Effie
    Oct 11, 2021 at 9:13
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    10.0.0.1 is a unique IP address regardless of the subnet mask. If you use it more than once, you’ve made a mistake.
    – Ron Trunk
    Oct 11, 2021 at 9:17

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Each IP address must be unique, no matter what its local subnet mask is or what prefix size is used on a route.

Let us say, one address in our subnet is 10.0.0.1 (subnet mask /8)

If you use 10.0.0.0/8 on any interface you can use addresses from that subnet only on that segment and nowhere else in your network.

Now, we decide to create a subnet further, and use 3 bits from the remaining 24 to form its network ID, thus creating a /11 network.

You mean you replace the /8 subnet with a /11 subnet that's been split off from the original prefix? OK.

Thus we have two host address that are same, albeit with the different network mask.

No. If you use a 10.0.0.0/11 subnet you cannot use 10.0.0.0/8 for a subnet anywhere else in your network any more. Doing so makes routing ambiguous and you don't want that.

My question is, does there exist any system to prevent this?

Not by default. It's your task as a network admin to prevent this (usually there's an address plan or IPAM in place). And usually you've got some kind of general logic that simplifies things. For instance, you could use /24 for all productive subnets, and you could structure subnets in supernets by location, purpose, security zone, ... like this:

  • 10.0.0.0/8 global scope
  • 10.1.0.0/16 location A
  • 10.2.0.0/16 location B
  • 10.3.0.0/16 location C
  • 10.x.0.0/24 client subnet on each location
  • 10.x.4.0/24 server subnet on each location
  • 10.x.8.0/24 IP phones on each location
  • 10.x.12.0/24 utitity devices/IoT
  • 10.x.16.0/24 security appliances (alarm, doors)
  • 10.x.20.0/24 SAN
  • 10.255.0.0/24 P2P link segment subnets /31
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