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Why do I get % Warning: use /31 mask on non point-to-point interface cautiously when I use a /31 subnet mask over Ethernet, and why am I not able to ping from R1 to R2 when both the interfaces have accepted the /31 subnet mask and they both are directly connected, and in which conditions can I use /31 without any trouble?

R1#show running-config interface fastEthernet 0/0
Building configuration...

Current configuration : 107 bytes
!
interface FastEthernet0/0
 ip address 216.21.5.1 255.255.255.254
 shutdown
 duplex auto
 speed auto
end

R2#show running-config interface fastEthernet 0/0
Building configuration...

Current configuration : 107 bytes
!
interface FastEthernet0/0
 ip address 216.21.5.2 255.255.255.254
 shutdown
 duplex auto
 speed auto
end

Topology:

enter image description here

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2
  • I had the same question about the warning, although I did not expect them to ping as there is no broadcast address available which I believe would stop the whole process. – Darth_Hamster Jul 27 '17 at 0:56
  • Actually, both addresses are broadcast addresses, and broadcast works because there is only one other device on the network. See RFC 3021, Using 31-Bit Prefixes on IPv4 Point-to-Point Links. – Ron Maupin♦ Jul 27 '17 at 0:59
11

Your problem is that you are addressing in two separate networks: 216.21.5.0/31 and 216.21.5.2/31. The addresses you are using are in the same /30 network, but two separate /31 networks.

You need to set one address as 216.21.5.0 and the other address as 216.21.5.1, or set one address as 216.21.5.2 and the other address as 216.21.5.3.

The warning is because ethernet is a broadcast technology which can have multiple endpoints, but a /31 network can only use two endpoints. If you were configuring it on a true point-to-point medium, I don't think you would get the warning.

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3

Using /31s is specified in RFC 3021 (Using 31-Bit Prefixes on IPv4 Point-to-Point Links).

Not all operating systems support this, especially Windows. And there are probably other system out there which do not support this RFC.

So using a /31 will not work with every connection and router gives you a friendly warning.

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0

Also, you could do "no shutdown" on relevant interfaces :)

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-3

Use a /30 prefix. A /31 prefix is not enough space for two hosts. You want R1 and R2 to be in the same network. A /30 network gives only two usable addresses which is enough for R1 and R2.

/30 = 255.255.255.252

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3
  • Using a /31 is perfectly acceptable for point-to-point links. It also gives you two addresses, and it doesn't waste two addresses like a /30 does. This was approved in December 2000 (15 years ago!) by RFC 3021, Using 31-Bit Prefixes on IPv4 Point-to-Point Links, and it is fully supported by Cisco. – Ron Maupin♦ Jan 22 '16 at 14:20
  • I created a gns3 topology with 2 routers and assigned a /31 mask, I'm surprised that worked out fine. I was always told that a /30 mask could only accommodate 2 possible hosts, because of the network address and broadcast address. – dsdk1987 Jan 27 '16 at 5:00
  • It's true, /30 can only accommodate two hosts, but it wastes two addresses for the network and broadcast addresses. That's why /31 was introduced 15 years ago. – Ron Maupin♦ Jan 27 '16 at 5:13

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