Hi,

Does /31 subnet mask support really exist in Linux? FYI, I am using the linux version 2.6.34.

As part of my trails, I have two linux boxes connected back to back. I am able to configure the interface addresses with a /31 netmask viz., 192.168.10.4 and 192.168.10.5 successfully. 'ifconfig' output confirms that. But I am not able to ping the other address from 192.168.10.4 and vice-versa.

So, I just doubt if this is really supported? If yes, Am I missing something?

Thanks in advance for any help in this regard.

i've always configured 2 host subnets as /30's then there's no conceptual issue here. I'm actually not sure whether a /31 subnet is meant to work according to RFC's (potentially regardless of reality). Does it work on a /30? (shifting the hosts up by one to fit nicely) Can you see any arps coming or going from a tcpdump?

I would say that if it works, congrats, but don't be at all surprised, or annoyed, if it doesn't. I just skimmed over the RFC from 2000 advocating it, and I don't like the idea of it outside of IP space conservation. Can you not use a /30?

an /30 network would probably be a better idea. if possible i would use it instead.

Thanks for your response, Chris. That was helpful.
I am in a necessity to work with /31's as part of my project work. It looks like it works with /30's. With /31's, I don't see any ARPs coming out on the wire. When we configure an interface with a /31 address one end of the link is a broadcast address. Is it because of some validations in the linux kernel code concerning the broadcast addresses that is preventing the (ARP/IP) packets to be sent out? If so, do we need to customize the kernel code to achieve this? Has anyone achieved success in making the /31's work?

If 30 is two ip addresses and 32 is one address then what is the point of a /31?


http://www.petri.co.il/how-30-and-32...networking.htm

Well there are RFC's to allow /31 on point-to-point connections.


But this is only discussed as specifically for p-t-p so on a normal ethernet interface I wouldn't expect it to work. Maybe you can set up a pppoe link here, that's probably the only way.

Thanks, Chris. Let me dig/try it out.

We're using /31 on all link networks between Cisco routers - directly connected as well as for IP tunnels. It works well and doubles the ammount of available link networks, which is good in respect to IPv4 depletion. So I would be pleased if it works on Linux too. Does anybody have any experience with it?

It works well with kernel 2.6.31 on SuSE 11.2:

The key here is the the following:
ifconfig eth0 pointtopoint <IPADDR>
This seems to work however I'm trying to work out how to hard code it...
I noticed via tcpdump without the above the interface will receive the traffic however the network stack doesn't seem to know how to send packets back.