nmap "Host x.x.x.x appears to be up" ... "(256 hosts up)"

[Erik Myrold <emyrold () gmail com>]

I am having an issue with a nmap host discovery scan (nmap -sP x.x.x.x/24) that is responding for 0 through broadcast 
255 when there are only 30 hosts on that subnet.

At this point I am not sure if it is the router or switch that is responding to the ping sweep.

What does this usually mean?  There is no NAT and no filtering that I can tell, but this is not my forte'...

There are other subnets I can ping sweep with no problems...

Thanks!