wha is ipv4 prefix?? (why not netmask)

[Miguel Angel Cañedo <mcanedo grupogcm com>]

I was pulling my hair trying to set static ipv4 settings.
Until I realized that NM 0.7 asks for PREFIX instead of NETMASK....

Now, my netmask should be 255.255.0.0
How do I transalte that into the Prefix?

What is that prefix thing?

Thanks,
Miguel