Originally Posted by RNDdave

Thanks. The app is called GNS3 which is a graphical network simulator. Basically lets me build and break router configs, useful for training (CCNA, CCNP etc) and useful for trying something in a safe environment before tampering with a working network...

It is really quite powerful and can be found HERE if you are interested. Getting it running on Mac is not quite as straight-forward as it is in Windows, but it's worth the extra few minutes effort tbh.