IP.Subnet.netmask

You're seeing just the function netmask, go back to IP.Subnet module for more information.

Specs

netmask(t()) :: IP.addr()

computes the netmask for a subnet.

iex> import IP
iex> IP.Subnet.netmask(~i"10.0.0.0/24")
{255, 255, 255, 0}