I believe nmap is suitable for scanning large number of IPs. I myself have scanned only /16 subnets with nmap but HD Moore in 2012 scanned the [entire Internet through nmap][1]. All you need is play with the command line arguments of nmap. The ones HD used for scanning the Internet were:

1. --min-rate=5000 -m 256 --min-host-group=50000 -PS -p
2. Match --min-rtt-timeout to --max-rtt-timeout

So I guess nmap can be quite effective. If it is capable of scanning the entire IP range over Internet, it won't have any problem fulfilling your requirements. [1]: http://critical.io/

I believe nmap is suitable for scanning large number of IPs. I myself have scanned only /16 subnets with nmap but HD Moore in 2012 scanned the entire Internet through nmap. All you need is play with the command line arguments of nmap. The ones HD used for scanning the Internet were:

1. --min-rate=5000 -m 256 --min-host-group=50000 -PS -p
2. Match --min-rtt-timeout to --max-rtt-timeout

So I guess nmap can be quite effective. If it is capable of scanning the entire IP range over Internet, it won't have any problem fulfilling your requirements.