def update_route_file(network, nat_server_ip, device)
      raise "ERROR: invalid nat_server_ip : '#{nat_server_ip}'" unless valid_ipv4?(nat_server_ip)
      raise "ERROR: invalid CIDR network : '#{network}'" unless valid_ipv4_cidr?(network)

      routes_file = routes_file(device)
      ip_route_cmd = ip_route_cmd(network, nat_server_ip)

      update_config_file(
        routes_file,
        ip_route_cmd,
        "Route to #{ip_route_cmd} already exists in #{routes_file}",
        "Appending #{ip_route_cmd} route to #{routes_file}"
      )
      true
    end
def update_route_file(network, nat_server_ip, device)
      raise "ERROR: invalid nat_server_ip : '#{nat_server_ip}'" unless valid_ipv4?(nat_server_ip)
      raise "ERROR: invalid CIDR network : '#{network}'" unless valid_ipv4_cidr?(network)

      routes_file = routes_file(device)
      ip_route_cmd = ip_route_cmd(network, nat_server_ip)

      update_config_file(
        routes_file,
        ip_route_cmd,
        "Route to #{ip_route_cmd} already exists in #{routes_file}",
        "Appending #{ip_route_cmd} route to #{routes_file}"
      )
      true
    end
Persist network route to file

 If the file does not exist, it will be created.
 If the route already exists, it will not be added again.

 === Parameters
 network(String):: target network in CIDR notation
 nat_server_ip(String):: the IP address of the NAT "router"

 === Return
 result(True):: Always returns true