Tips and Concepts

Using /31 or 254 Subnet

As we are running out of IPv4 address space on the Internet, it makes sense to consider a subnet where minimum IP address wasted as possible.
On a 30-bit subnet mask on a Point-to-Point link, which requires four addresses per link. A would yield:

One for (all-zeros) network ID (
Two for host addresses ( and
And one for (all-ones) broadcast (

This unfortunate point-to-point links wasting 2 addresses from the address block. The network ID and the broadcast addresses.

In December 2000, IETF propose the RFC 3021 - 31-Bit Prefixes on IPv4 Links allowing us efficiently make uses of the existing address space. One side will hold the network-ID address while the other hold the broadcast address.

Cisco IOS 15 and above support RFC 3021 allowing us to create more point-to-point network for a given subnet such as: can have 128 subnets of /31 mask can have 8 subnets of /31 mask

The following image show a configuration and output of a /31 subnet.

Here is an output of routing table for 31-bit mask: