The TCP/IP Guide - IP Variable Length Subnet Masking (VLSM)
The Sky Networks - CIDR CIDR (Classless Inter-Domain Routing) notation rather than the common subnet mask 255.x.x.x when configuring addresses and networks. Refer to the CIDR Subnet Table to find the CIDR
Ability to use Slash notation (CIDR) instead of subnet mask - Feature Requests - Network Performance Monitor (NPM) - THWACK
Understanding IP Addressing and CIDR Charts — RIPE Network Coordination Centre
Cisco Slash Notation - FlackBox
The TCP/IP Guide - IP Subnetting Step #3: Determining The Custom Subnet Mask