If you use the separate tier topology, where the application server instances and HADB nodes are on separate host machines, you can improve performance by having all HADB nodes on a separate subnet. This is because HADB uses the User Datagram Protocol (UDP). Using a separate subnet reduces the UDP traffic on the machines outside of that subnet. Note, however, that all HADB nodes must be on the same subnet.
You can still run the management client from a different subnet as long as all the nodes and management agents are on the same subnet. All hosts and ports should be accessible within all node agents and node must not be blocked by firewalls, blocking of UDP, and so on.
HADB uses UDP multicast, so any subnet containing HADB nodes must be configured for multicast.