The TCP/IP Guide - IP Subnetting Step #3: Determining The Custom Subnet Mask
Step by Step Guide to Understanding IP Subnetting - Expert Network Consultant
Subnet Mask Validation | RedNectar's Blog
Consider that the IP address is 192.248.87.3 and the sub-net mask is 255.255.255.224. How many hosts can be directly connected to this network? - Quora
Subnet mask tables | Article about subnet mask tables by The Free Dictionary