find all active IP addresses in a network

nmap -sP 192.168.1.0/24; arp -n | grep "192.168.1.[0-9]* *ether"
You send a unicast ICMP packet to each host. Many firewalls will drop that ICMP. However, in order to send the ICMP, you'll have first done an ARP request and the remote machine is unlikely to ignore that, so the computer will be in your ARP table.
Sample Output
172.16.17.99             ether   00:0b:db:7f:c1:4c   C                     eth0
172.16.17.203            ether   00:15:60:9a:76:71   C                     eth0
172.16.17.80             ether   00:15:60:5a:6b:0c   C                     eth0
172.16.17.142            ether   00:12:3f:27:a1:34   C                     eth0
172.16.0.12              ether   00:1e:c9:b0:3a:96   C                     eth0

14
By: gavinmc
2010-04-12 14:36:15

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