Openflow is an "open" protocol to enable communication between a Software defined Network controller and network elements. Openflow enables the controller to create flow table entries in the switches, and also enables the analysis for underlying network topologies. The Mininet provides a simple and inexpensive network test-bed for developing Openflow applications. The default version of the Mininet supports pre-defined topologies such as single, linear, tree etc., in a single IP network. To generate custom topology, one has to write programs in the Mininet. The goal of the proposed work i s to provide a custom topology framework which enables the creation of any custom network topology of user's choice, including multiple IP networks. Thus, user can carry out research on different networks having separate broadcast domains. The framework has been so designed that it will not affect the performance of t he ex isti ng Mininet significantly.