Thanks for the reply Karlo !
Incorrect octet was one of the issues, the other one was my thunderbolt ethernet on mac playing up. After removing it and adding it again, restarting VIRL, I managed to get connectivity between my Mac terminal and a router on FLAT-1.
After this, I expected I will be able to get to other nodes in my topology through added routing, as I would in a physical network.
Is it the case that each node needs its won flat network and is there a smart way to have another VM (ubuntu) reaching all nodes in VIRL VM ?
Thanks in advance