Code check request

Sure I got a couple tips :)

It is common to use atoms as keys in keyword lists and maps instead of strings. Some syntax is only available if you do, and it will have a side benefit of making you programs slightly smaller / faster ;)

in compare_ips the cond would look better as an if

so like this:

def compare_ips(old_ip, opts) do
  if old_ip == opts[:ip] do
     { :unchanged, opts }
  else
     { :changed, %{opts | old_ip: old_ip } }
  end
end

My preference would be to reduce one more step and do the matching in the function head, like so:

def compare_ips(ip, %{ip: ip} = opts), do: {:unchanged, opts}
def compare_ips(old_ip, opts), do: {:changed, %{opts | old_ip: old_ip)}

First thing, your spacing is way off. Idiomatic Elixir uses 2 spaces and no tabs, check your editor is setup right.

Second, it seems what you want from your PubIpMon.CLI module is a mix task. Read about mix tasks here.

Also, I suggest you keep your functions which cause side effects (in this case the ones that do IO.puts) to a minimum so that those functions are easier to test.

Try looking at using the new with keyword in Elixir 1.2 if you're brave and remove all the IO.puts from all functions except PubIpMon.Comparator.compare/2.