There is a permanent need for planning and analysis of existing networks, provisioning of new demands, installing new components in the network, and study possible networking options. To fulfill all these requirements, a flexible design tool is needed, that is based on a flexible network model. FLEXPLANET, the presented modeling, design and analysis tool makes it possible to consider real size operating networks using existing technologies, while supports the introduction of future networking options. The main properties that provide the flexibility are: (i) the separation of the network model and the data structures, that the algorithms work on, (ii) the object oriented design of the model, (iii) the generic server-client hierarchy description realized with the concept of extension.