Scan for connected, powered on computers on a network?

Solution 1:

Have you tried other scan options from nmap? The ping option may not always work, but other scan options may be more reliable? Try,

nmap -PS 192.168.2.0/24 (TCP SYN ping)

or

nmap -PR 192.168.2.0/24 (ARP scan)

Solution 2:

Try Look@LAN to scan for powered on computers.

There are alternatives to Look@LAN and some of them are even free or free and open source.
Personally I can recommend the Angry IP Scanner which is OpenSource and multiplatform or Advanced IP Scanner which is made for Windows platforms and not OpenSource but free.

Solution 3:

ping -b the broadcast IP then arp -a

Solution 4:

It seems that the problem was the probing rate (too much packets per second?). Using the parameter --max-rate made it performance much better. It takes ages to run, but at least it works.

$ nmap -sP --max-rate 1  192.168.2.0/24
...
Nmap done: 242 IP addresses (67 hosts up) scanned in 434.04 seconds