We use ipfire to run a /24 network on green and want to connect to another router on red via a 192.168.0.24/31 transfer net, so red is assigned to 192.168.0.24, connecting to the other routers IP 192.168.0.25, with no other IPs within this transfer net.
We tried and failed. We cannot even ping the other router from red.
Using a 192.168.0.24/30 transfer net works fine with red on 192.168.0.25 and 192.168.0.26 for the other router.
Is there a reason, why /31 transfer nets cannot be used or is there any other problem we haven’t tought of?
Each net a.b.c.d/n defines a net address with d=min ( least significant 32-n bits = 0 ) and broadcast address with d=max ( least significant 32-n bits = 1 ), which can’t be used as client IPs.
The net 192.168.0.24/31 has net address 192.168.0.24 and broadcast address 192.168.0.25.
The net 192.168…0.24/30 ( net address 192.168.0.24, broadcast address 192.168.0.27 ) gives the client IPs 192.168.0.25 and 192.168.0.26.
Hello Bernhard,
until this posting from @huebler i thought the very same.
Here comes what i found. That was totally new for me. So there is a chance that i have misinterpreted the information behind the link. Iam interested what you think about.
https://tools.ietf.org/html/rfc3021
Edit:I tried to address you with this. This was probably not clearly expressed before.
Thanks for the answers so far. We allready knew about rfc3021. Our network department initially assigned us a /31 transfer net with public ip addresses, saying that they are using this all over the place without any problem. We also tried to configure 31-bit network masks on debian linux and it works well. It’s even possible to configure 31-bit masks on ipfire, it just doesn’t work.
I don’t know all network related sources exactly, but I suppose RFC 3021 isn’t realized in IPFire.
The main problem in the moment may be that all ( /31 nets also ) are used with a network and a broadcast address. These aren’t necessary in a net with only two hosts ( to reach all hosts in the net, it is sufficient to send to the other host, … ), but IPFire isn’t aware of this ( yet ).
Could you please open a ticket in Bugzilla? Thus the problem can’t get lost.
The smallest possible network is /30 because /31 has no room for IP addresses.
/30 has two free addresses + network and broadcast address so it is the smallest possible subnet.
Hello Arne,
thats what Bernhard and i already thought. But after reading the info above and thinking about, thats atleast for me not so sure anymore. Because why should i need a BC and a Net ID if only 2 talks to each other? so for me is after this rfc the smallest possible subnet /31 for 2. Whatever… since this reading about this rfc iam still confused about 
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The network & broadcast addresses are effectively “reserved” and not available for hosts. Thus an additional 2 addresses are needed for the minimal 2 host network - that requires /30
Thanks to all for contributing.
Seems that we have a discrepancy between what rfc3021 suggests and what ipfire does.
I opend a ticket in Bugzilla 12486
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