IP.prefix

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

Specs

prefix(v4(), 0..32) :: v4()
prefix(v6(), 0..128) :: v6()

generates an the subnet prefix for a given ip address.

iex> IP.prefix({10, 0, 1, 23}, 24)
{10, 0, 1, 0}