nmap does not find alive hosts with ping scan

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http://serverfault.com – I try to ping scan my local network using nmap, but it doesn't seem to find machines that are for sure alive. It responses to ping but on to ping scan. Results for ping: $ ping PING ( 56 data bytes 64 bytes from icmp_seq=0 ttl=64 time=1.585 ms And output of tcpdump: $ tcpdump -i fxp0 -n host 12:43:07.624911 IP > ICMP echo request, id 61023, seq 0, length 64 12:43:07.625029 IP > ICMP echo reply, id 61023, seq 0, length 64 But when I try to use nmap it doesn't see it i (HowTos)