What are the consequences of having a subnet within a subnet? Eg. First network is and the second is

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 network because of unreturned responses from the network's devices. Any ideas or am I crazy?

  • 1
    could you give us some more information about how the subnets are routed and where the subnets exist? A diagram would be great Commented Oct 24, 2013 at 20:37
  • This is a flat switched network. There are a number of devices that exist in the network, and a number that exist in the network.
    – David
    Commented 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? Commented 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
    Commented Aug 8, 2017 at 14:43

5 Answers 5


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

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


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

0 is a classfull network. 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.

  • 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. Commented Oct 25, 2013 at 3:44

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.


Well, and overlap each other. does well if you're using it to refer to other networks you have within that range.

However if you have addresses in that spill over in range then they can't communicate.

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