In this paper, we study the hardness of the IP-subnet aware routing problem in WDM network. IP-subnet aware routing attempts to reduce routing overhead by grouping a set of addresses under a single subnet. However, routing in WDM network with subnets imposes neighboring IP interfaces along the path to be on the same subnet. This subnet constraint forces the routing path to be longer unless routing permits reconfiguration of subnets. The hardness of this routing problem is first studied in [1]. We investigate further the problem of finding paths accounting for subnets and its hardness. Regardless of subnets, we argue that a shortest IP hop path can be constructed in a polynomial time. We provide efficient routing algorithms to justify our argument.