# Pastebin FCR5iIgh
let
  pkgs = import <nixpkgs> {};
  lib = pkgs.lib;

  data = {
    host-a = {ip = "1.1.1.1";};
    host-b = {ip = "2.2.2.2";};
    container = {
      host-c = {ip = "3.3.3.3";};
      host-d = {ip = "4.4.4.4";};
    };
  };

  build-hosts = sets: path: res:
    if sets ? ip then
      { "${sets.ip}" =  (lib.concatStringsSep "-" path); }
    else
      lib.foldr (key: out:
        out // (build-hosts sets."${key}" (path ++ [key]) res)
      ) res (builtins.attrNames sets);
in build-hosts data [] {}