Parallel Host Scanning
Multiple hosts can be specified on the nmap command line, and nmap's efficient scanning processes will often probe these hosts simultaneously to provide the most efficient scan possible. However, there may be instances when the minimum or maximum number of parallel host scans should be modified.

Maximum Parallel Hosts per Scan (--max_hostgroup <number>)
Nmap is extremely efficient when scanning groups of hosts simultaneously. Unfortunately, this efficiency also requires that nmap complete the scanning of a host block before the results can be displayed. At other times, it may be more appropriate to scale the number of simultaneous hosts to a large number to improve the efficiency during a batch scanning process. By adjusting the --max_hostgroup option, the number of parallel host scans can be modified to suit any situation.

Many port scans support scanning hosts in parallel, but not all scan types will make use of this option. For example, the ping scan (-sP) uses its own grouping algorithm and completely ignores the --max_hostgroup option.

The default maximum host group size is 100,000 hosts. This is set intentionally high so that the scan will not be restrictive unless the user manually sets a maximum value.

Minimum Parallel Hosts per Scan (--min_hostgroup <number>)
The minimum number of parallel hosts to scan option (--min_hostgroup) allows the nmap scan to maintain a high level of efficiency and throughput when scanning a large group of IP addresses in unattended batch processes. This option forces nmap to scan at least this number of hosts in parallel, although these simultaneous scans will require more memory than smaller host groups.

This minimum value must not be larger than the maximum (--max_hostgroup), or nmap will provide an error message and cancel the scan. The default number of minimum hosts is one.

If --min_hostgroup is configured with more than 100 simultaneous host scans, nmap will provide this warning message:
Warning: You specified a highly aggressive --min_hostgroup.
This large group may be required, but nmap provides this sanity check to make sure you know what you're getting into.