If we are adding one IP / Host per subnet then that line speed will be applicable for the one IP / Host. If we add a range of IPs / Hosts under subnet, then the Line Speed mentioned will be applicable per IP / Host. E.g., my IP range is 192.168.43.2 --- 192.168.43.254 and the Line Speed mentioned under Realism tab is 10. So, for each IP starting from 192.168.43.2 to 192.168.43.254 the Line Speed will remain the same which is 10.