-sP    Ping scanning: Sometimes you only want to know which hosts on  a
              network  are  up.  Nmap can do this by sending ICMP echo request
              packets to every IP address on the networks you specify.   Hosts
              that   respond  are  up.   Unfortunately,  some  sites  such  as
              microsoft.com block echo request packets.  Thus  nmap  can  also
              send a TCP ack packet to (by default) port 80.  If we get an RST
              back, that machine is up.  A third technique involves sending  a
              SYN  packet  and  waiting  for a RST or a SYN/ACK.  For non-root
              users, a connect() method is used.

              By default (for root users), nmap uses both  the  ICMP  and  ACK
              techniques  in parallel.  You can change the -P option described
              later.