RFC 3021: the "/31" subnets for IPv4 Point-to-point Links

(This RFC has been there for almost 14 years. Without writing a post down about it, it would never stay in my mind as if I have never known it. Too bad for me!)

Original location of West Gate of Taipei City Walls (西門、西門町).
The gate and the walls were removed long time ago.
The bottom right is one of the entrance of  MRT Ximen (e.g. West Gate) Station.

RFC 3021: Using 31-Bit Prefixes on IPv4 Point-to-Point Links

Since this RFC is quite old, almost all of today's routers already have this feature. It is very safe today to assign "/31" subnets to any point-to-point links. We can now save 50% of IPv4 addresses for point-to-point WAN links.


For example, assume I have a range of available IPv4 addresses in "192.168.1.0/24" for point-to-point WAN links. With "/30" subnetting scheme, I can only assign to 64 point-to-point links.

To visualize it, the first address assignment of such "/30" is: 192.168.1.0, 192.168.1.1, 192.168.1.2, and 192.168.1.3. Addresses 192.168.1.1 and 192.168.1.2 are assigned on two routers connected with the point-to-point link. Addresses 192.168.1.0 (network address) and 192.168.1.3(broadcast address) are reserved.

192.168.1.0 (Reserved, unusable, wasted)
192.168.1.1 (Usable, Router 1)
192.168.1.2 (Usable, Router 2)
192.168.1.3 (Reserved, unusable, wasted)

With RFC 3021, the new "/31" scheme, I can now assign the same range to 128 point-to-point links. This new assignment scheme saves 50% of required IPv4 addresses!

To visualize it, the first address assignment of such "/31" is: 192.168.1.0 and 192.168.1.1. No more reserved "network address" and "broadcast address" are wasted!

192.168.1.0 (Usable, Router 1)
192.168.1.1 (Usable, Router 2)