IP.Subnet.is_in

You're seeing just the macro is_in, go back to IP.Subnet module for more information.
Link to this macro

is_in(subnet, ip)

View Source (macro)

true if the ip parameter is inside the subnet. ip must be a single ip address; if you need a membership function that accepts ranges or subnets, use Kernel.in/2.

Be aware of the parameter order, if you are using this after import IP.Range.

Currently only works for ipv4 addresses.

usable in guards.

iex> import IP
iex> IP.Subnet.is_in(~i"10.0.0.0/24", ~i"10.0.0.2")
true
iex> IP.Subnet.is_in(~i"10.0.0.0/24", ~i"10.0.1.5")
false