Configuration Example for 31-Bit Subnet Mask Configuration

31-bit subnet masks can be used to conserve address space. In the following diagram, Router A is connected to Router B as a point-to-point link with 10.1.1.0/31 subnet. There are only two available addresses in this subnet, 10.1.1.0 on Router A and 10.1.1.1 on Router B.

Routers B and C are connected by a regular 24-bit subnet. Router C can either be a switch with many hosts belonging to the 10.2.2.2/24 subnet connected to it, or it can be a router.

Configured 31- Bit and 24-Bit Subnet Masks

Router A

RouterA(config)# interface ethernet 1/1/1
RouterA(config-if-e1000-1/1/1)# ip address 10.1.1.0/31

Router B

RouterB(config)# interface ethernet 1/1/1
RouterB(config-if-e1000-1/1/1)# ip address 10.1.1.1/31
RouterB(config-if-e1000-1/1/1)# exit
RouterB(config# interface ethernet 1/3/1
RouterB(config-if-e1000-1/3/1)# ip address 10.2.2.1/24

Router C

RouterC(config# interface ethernet 1/3/1
RouterC(config-if-e1000-1/3/1)# ip address 10.2.2.2/24
Parent topic: 31-Bit Subnet Masks on Point-to-Point Networks