Tips and Concepts


 31-bit Mask         Tips and Concepts

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:

Two host addresses ( and
One all-zeros network ID (
One 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

Optimize OSPF /30 network  
Layer-2 Resolving  
Data Encapsulation  
"Fish-Mo" of AD  
Layer-2 Redundancy  
Using /31 Subnet  
FSMO: Inf Master and GC  

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

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