Scan finishing on 3.8k hosts after a few seconds

I feel like this question may have been answered several times, but I just can’t find out where. When I go to queue up a scan, it seems to finish in just a few seconds. Here’s what I see in the logs:

libgvm boreas:MESSAGE:2021-07-28 23h42.19 utc:8207: Alive scan 85c9d71d-34fc-4340-8b7f-8726a06ca203 started: Target has 3840 hosts                                                                                                                                                                                                                
libgvm boreas:MESSAGE:2021-07-28 23h42.28 utc:8207: Alive scan 85c9d71d-34fc-4340-8b7f-8726a06ca203 finished in 9 seconds: 0 alive hosts of 3840.                                                                                                                                                                                                 
sd   main:MESSAGE:2021-07-28 23h42.29 utc:8207: Vulnerability scan 85c9d71d-34fc-4340-8b7f-8726a06ca203 finished in 14 seconds: 0 alive hosts of 3840

However, if I trim this list down to maybe 30 systems, the scan takes much longer. Does this mean that there’s some kind of cap on the number of systems that can be provided to the scan or something?

Even if those systems aren’t active, how is it determining 3.8k systems aren’t active in just 14 seconds?

Most likely your firewall or network device is blocking the alive tests.

Thanks @bricks for the great help as usual. Ran Nessus and turns out it ran for about 2 days and had no vulnerabilities, so I definitely agree here.

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