What is a subnet mask - an IP address followed by a slash and a number?
This refers to a group
of IP addresses. The group comprises all of the addresses where the n leftmost bits (where n is the number appearing after the slash) are as in the given IP.
For example, for a specific purpose the following IP addresses should be used: 100.64.0.0/10
. What this means is that only IPs that start with the top 10 bits of 100.64.0.0 should be used (those 10 bits being: 0110010001). Since IPs are 32 bits long, there are 22 bits of available IPs for use (i.e. over 4 million IPs) from 100.64.0.0 to 100.127.255.255. [The specific purpose being for Carrier-Grade NATs - CGNs. See RFC 6598