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My bare knuckles networking understanding led me to believe that if I set an IP address of, for example, 10.0.2.25 with netmask 255.255.0.0 on a machine, it should see all hosts in the subnet 10.0.1.0/24 (and many others on the whole 10.0.0.0/16 subnet)

I just tried it, but the machine with 10.0.2.25/16 didn't see at all 10.0.1.0/24 addresses.

Two questions:

  1. How to set up such a network simply?
  2. What's the mistake in my understanding?
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possible duplicate of How does IPv4 Subnetting Work? –  Tom O'Connor Apr 3 '13 at 10:47
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@TomO'Connor I'm not sure this practical question is answered directly in the canonical answer about IPv4 subnets. –  Vinko Vrsalovic Apr 3 '13 at 12:38
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3 Answers 3

up vote 9 down vote accepted

I just tried it, but the machine with 10.0.2.25/16 didn't see at all 10.0.1.0/24 addresses

10.0.2.25/16 can send packets to 10.0.1.0/24, but when 10.0.1.0/24 tries to respond it thinks the opther host is outside it's network hence will try to send the packets back through a router (or fail).

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How to set up such a network simply?

Just as you did, you got it right in principal, there must just be an issue with the implementation - certainly there's nothing wrong with the idea of doing what you did.

What's the mistake in my understanding?

Nothing, you just need to ensure that this use of a /16 is implemented everywhere appropriate, on every machine in that range, in every router etc.

Use of non-/24 netmasks is extremely common these days and works great, but you do need to ensure they're properly, and thoroughly, implemented.

Otherwise good work ;)

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Obviously, if the other machine is a /24, it can only see machines in that scope. The subnet masks need to match if they are going to be visible to one another.

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