Question about networks. What IP will be? What kind of masks and stuff?

Question

Computer networks

Hello!
Suppose there is a scheme:
Router 1 -> Switch 1 -> [PC 1, PC 2, PC 3]
Router 2 -> Switch 2 -> [PC 4, PC 5]
Well, the IP address of this network, for example: 1.2.3.64 /27.
What does the "27" after the slash mean and how to determine the IP of devices in this case?
And what about device "masks"?
Help me figure this out, please.

Answer 1

Equivalent to 255.255.255.224.
In this case, the pool of ip addresses will be as follows:
from 1.2.3.65 to 1.2.3.94.
Also, for the calculation there is a good site Zhmyak