Function is_nameable_host 

pub fn is_nameable_host(host: &str) -> bool

Is this a host the net.out(host) atom can name?

The atom is written in a uses clause as bare tokens — net.out(payments.example.com) — so a host a program calls has to be one a program can declare. Dotted ASCII labels, which is what a DNS name is and what a NetworkPolicy peer is written as. No port: a port is transport, and the policy peer is the name.