nmap scan question (identifying hosts that are alive)

[alternaloser]

Hello,
I've went through quite a few iterations to try and find out a way to do this. Essentially pinging to find live hosts doesn't work on all systems in my network, as with many. I'm wondering what will output all systems that have an open port of any sort, or some other way to identify living systems. The end result of this data is being scanned with Nessus. Anyway, the command line I ended up having to use was
nmap -iL netranges.txt -F -O -P0 > NETWORKnmap.txt

This resulted in every IP in every network range being listed as a server, but most of the other scans I attempted didn't find anything. How can I excise entries like this:

Nmap scan report for 10.xx.xx.xx
Host is up (0.22s latency).
All 1000 scanned ports on 10.xx.xx.xx are filtered
Too many fingerprints match this host to give specific OS details

yet still find every single live host on the networks I'm scanning?

[alternaloser]

Oh and the -O switch is not 100 percent necessary I suppose since it takes quite a bit longer and with the other options I'm using it doesn't really have any ability to identify the OS, but as I'm trying to run a Nessus scan afterwards if I can somehow identify the OS and therefore run the correct sets of Nessus policies, that would be amazing. The important thing here, for me, is to only get living systems in the end results, preferably identified by OS.

[alternaloser]

One thought I had was I could run a scan that simply identifies all of the systems, then a second that does a more in-depth port scan to try and identify OS. I don't know, I haven't used nmap in some time and I've never really used it to its full potential.

[Kloschüssel]

There's still the --osscan-limit switch for OS detection. In general I would strictly specify which ports to scan for with the -p switch, thus scanning only about 10 or 15 interesting ports depending on the applications you expect to run. It should speed up the thingy a lot. Does this answer your question?