A simple example explaining why we need Prefix-List in addition to simple Access-List (ACL) on Cisco IOS

My example is: assume we want to filter out "all possible subnets/prefixes inside 192.168.1.0/24" from rushing into our router.

Red House Theater, at the West Gate of Taipei Wall (西門紅樓、紅樓劇場). 

We can first visualize what subnets are to be filtered in the following, but incomplete list:

192.168.1.0/24

192.168.1.0/25
192.168.1.128/25

192.168.1.0/26
192.168.1.64/26
192.168.1.128/26
192.168.1.192/26

(and even more ...)

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.