What does “192.168.1.1/21” mean?

Question

When they write a slash followed by a number after an IP address, what does that mean?

Answer 1

It's CIDR notation for a subnet.

The /21 denotes that the first (most-significant) 21 bits are common to all addresses in the subnet. The individual hosts in the network have addresses that are different only in the last 11 bits.