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What are the consequences of having a subnet within a subnet? Eg. First network is 10.0.0.0/8 and the second is 10.0.1.0/24.

Thanks for everyone's replies. The original design, which is not my own, was to simply have two separate networks that have no need to communicate with each other. For some reason the tech that set it up did it this way. I realize that they are separate subnets and therefor unable to fully talk without routing. Just a little confusing that they were setup within each other.

My main concern though is if this would degrade performance within the 10.0.0.0/8 network because of unreturned responses from the 10.0.1.0/24 network's devices. Any ideas or am I crazy?

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    could you give us some more information about how the subnets are routed and where the subnets exist? A diagram would be great
    – Mike Pennington
    Oct 24, 2013 at 20:37
  • This is a flat switched network. There are a number of devices that exist in the 10.0.0.0/8 network, and a number that exist in the 10.0.1.0/24 network.
    – David
    Oct 24, 2013 at 20:39
  • So you are saying that the net masks on various machines in the VLAN are mismatched, right? Which operating systems are on the subnet?
    – Mike Pennington
    Oct 25, 2013 at 13:38
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Aug 8, 2017 at 14:43

5 Answers 5

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The hosts in 10.0.1.0/24 will not be able to communicate with any other hosts in any other subnets of 10.0.0.0/8 without a default-gateway.

The hosts in 10.0.0.0/8 will be able to communicate with all hosts in 10.0.0.0/8, which includes 10.0.1.0/24, but only one-way if the hosts in 10.0.1.0/24 do not have a default-gateway with access to both subnets.

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The short answer is that the two subnets will not be able to communicate. The 10.0.0.0/8 subnet will believe that the 10.0.1.0/24 subnet is local. Thus, traffic from 10.0.0.0/8 to 10.0.1.0/24 will never be routed (it will stay local). Traffic from 10.0.1.0/24 to 10.0.0.0/8 will be routed to the default gateway. This traffic will make to the desired host in the 10.0.0.0/8 subnet; however, return traffic (required for TCP) will not be returned. In theory, UDP from 10.0.1.0/24 to 10.0.0.0/8 will be successful. However, if there is any requirement at the application level for acknowledgements (TFTP, etc.), the application will fail.

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10.0.0.0/8 is a classfull network.
10.0.1.0/24 is a subnet of /8 classfull network.
After subbneting you're creating a range of address that are separated logical.
Host from /8 network will not be able to communicate with host from /24 subnet without router.

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  • Hosts in the /8 will be able to communicate with hosts in the /24 without a router, but the /24 guys won't know how to get back to the /8 dudes without a router, resulting in one-way asymmetric IP communication.
    – Yosef Gunsburg
    Oct 25, 2013 at 3:44
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David, depending on your setup this might work and is a known way of directing traffic to the most-specific prefix.

You could announce the /8 and the /24 from two different interfaces and this would shove the traffic that wasn't in the /24 into one path and the /24 to another.

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Well, 10.0.0.0/8 and 10.0.1.0/24 overlap each other.

10.0.0.0/8 does well if you're using it to refer to other networks you have within that range.

However if you have addresses in 10.0.0.0/8 that spill over in 10.0.1.0/24 range then they can't communicate.

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