What would happen if a point-to-point link is addressed with a /31 subnetmask? In networking, when a point-to-point link is mentioned, what comes to mind is a /30 subnetmask. The use of the /30 subnetmask is generally seen as a move geared towards the conservation of IP addresses. This is because a point-to-point link only requires two IP addresses which a /30 subnetmask provides in addition to the sunet and broadcast addresses.
For most readers, looking at the network topology below, the first thing that comes to mind is ” no way that is going to work.” Well, judging from what was learned in IP address subnetting, such readers could be correct. This is because a /31 subnetmask gives two IP addresses. The ultimate question therefore is, are the two IP addresses provided by a /31 subnetmask usable?
Looking at a /30 subnetmask, it provides for a block of four IP addresses out of which only two are usable. The remaining two are the subnet and the broadcast addresses. Remember that in subnetting, you are to subtract two from the total number of host in any subnet. Therefore, the number of host for a /31 subnetmask will be (2^1)-2=0. Can we then say that based on the result of the calaculation above, a /31 subnetmask is unusable since it provides no host address? The answer lies in knowing whether a broadcast is needed on a point-to-point link. The answer is no. If broadcasts are not needed on point-to-point links, it therefore means that a /31 subnetmask can be used. But will the router accept such addressing scheme?
In the lab below, we will see that the /31 subnetmask can actually be used on a point-to-point link.
On R1, let’s see if the addressing scheme on the topology will be accepted by the router. See commands below:
R1(config)#interface Serial0/0 R1(config-if)#ip address 10.1.1.0 255.255.255.254 R1(config-if)#no shut R1(config-if)#end
On R2, we repeat thesame thing using corresponding IP address and interface. See commands below:
R2(config)#interface Serial0/0 R2(config-if)#ip address 10.1.1.1 255.255.255.254 R2(config-if)#no shut R2(config-if)#end
I will verify by running a ping from R1 to R2. See result below:
The image above shows the ip configured on interface serial 0/0 as well as the result of a ping test to 10.1.1.1, the IP address on R2’s serial interface 0/0.
In conclusion, from the result of this lab, we can say that using the /31 subnetmask for point-to-point links is preferable since no IP addresses are wasted.
This lab was simulated on GNS3 using two Cisco 3745 ISR routers.