[openstack-dev] [neutron] Neutron should disallow /32 CIDR

CARVER, PAUL pc2929 at att.com
Thu Jan 23 20:22:06 UTC 2014

Paul Ward:

Thank you to all who have participated in this thread.  I've just proposed a fix in gerrit.  For those involved thus far, if you could review I would be greatly appreciative!


I wouldn't go so far as to say this verification SHOULDN'T be added, but neither would I say it should. From a general use case perspective I don't think IPv4 subnets smaller than /29 make sense. A /32 is a commonly used subnet length for some use cases (e.g. router loopback interface) but may not have an applicable use in a cloud network. I have never seen a /31 network used anywhere. Point to point links (e.g. T1/Frame Relay/etc) are often /30 but I've never seen a /30 subnet for anything other than connecting two routers.

However, does it really benefit the user to specifically block them from entering /32 or block them from entering /30, /31, and /32?

It might not be an equal amount of code, I think a much better effort to help the user would be to provide them with a subnet calculator directly in Horizon to show them how many usable IPs are in the subnet they're defining. In this case, displaying "Usable addresses: 0" right when they enter /32 would be helpful and they would figure out for themselves whether they really wanted that mask or if they meant something else?

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