nmap -sn -n --unprivileged

note: "unprivileged" is important because if you omit it, nmap tends to
spurious report almost every host as "up". it does this because it uses raw
sockets and will interpret some firewall response packets as ping responses
from the host; if you use unprivileged, nmap is forced to use the normal
system calls for sending a ping and will only get a reply if there really is