Using /31 subnets on point-to-point links
A recent post on /30 and /31 addressing prompted one reader to ask when 31-bit subnet masks are appropriate, or supported. It turns out they've been supported for a while.
Thirty-one-bit subnets were first proposed in RFC 3021, which was primarily motivated by the potential for public address space conservation. Recall that shrinking a /30 subnet to a /31 effectively doubles the number of point-to-point links you can address from a finite range. Cisco IOS has supported /31 subnets for point-to-point links since release 12.2(2)T. A thorough explanation of the feature is presented here.
We can put this theory into practice by addressing a point-to-point connection between two routers as 10.0.0.0/31. Note that this even applies to Ethernet interfaces, technically a broadcast medium.
Router(config)# interface f0/0 Router(config-if)# ip address 10.0.0.0 255.255.255.254 % Warning: use /31 mask on non point-to-point interface cautiously
An ominous warning message, no doubt, but it works just fine. We can successfully ping the far-end interface (10.0.0.1), and the subnet is accurately reflected in the routing table:
Router# ping 10.0.0.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/16/20 ms
Router# show ip route
...
10.0.0.0/31 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, FastEthernet0/0
About the Author
Jeremy Stretch is a networking engineer and the maintainer of PacketLife.net. He currently lives in the Raleigh-Durham area of North Carolina. Although employed full-time out of necessity, his true passion lies in improving the field of network engineering around the world. You can contact him by email or follow him on Twitter.
Comments
Nice , never knew that Before
Cool. But what happens if there is similar route but with a /30 subnet mask in the routing table?
I think a directly connected network would win
Good one. thanks.
any known problems with using /31 subnets?
We use /31's almost exclusively in our very large international network. No problems discovered after years of usage.
"Cool. But what happens if there is similar route but with a /30 subnet mask in the routing table?" before we even get into the connected route winning, the longer match would be preferable thus the /31 would win
This goes against my CCNA training, I've been living a lie!
Sooooo wrong to be able to ping .0 !
This probably never caught on with providers because there was no mechanisim to roll it out whout readressing your existing customers' circuits. If you are using .0/20, then your usable hosts are .1 and .2, then .5 and .6 in the next block. /31 opens up .0/.1, .2/.3, /.4/.5. . . .If there was a way to keep your existing endpoint IPs and make available p-p circuits of .3/.4, so you could just change masks and then add circuits in the "dead space"
Think, not only of all the customer reconfiguration, but route table overhauls.
It was a good idea in theory.
Is there anyway to have this work up on an ASA? I've used it with sucess for point-to-point links on routers etc, but seems like both pix's and ASA's throw a "ERROR: /31 mask is not allowed"
I've raked for docs, and cant find anything!
(By the way, Loving the CAPTCHA, reduce spam, reduce fools :D )
COol...cisco rollout.
Very nice. Never seen it in production networks yet, thou !
We use it in our network on many link. Will not mention company name but I will say we have over 9000 stores.
What about GARP reply.
Hi,
check this excellent case study on 'Use of ‘/31 IP subnet’ on point to point links; and their effect on routing protocols'
Nice, but I have never seen this in a production network!