Addressing II - What are netmasks for?

This video is part of a set of videos that explain how addressing works in IPv4.
You can find the link to all of them below :)
Addressing I: https://youtu.be/7KkQgKYFMbs
Addressing II: https://youtu.be/fvxdyMx9S-U
Addressing III: https://youtu.be/0ZrXtdZu5Ac
Addressing IV: https://youtu.be/L4xz-OZTQF4
Addressing V: https://youtu.be/ZU9dO9PIRBk