Open the Command Prompt and type in the following:
FOR /L %i IN (1,1,254) DO ping -n 1 192.168.10.%i | FIND /i "Reply">>c:\ipaddresses.txt
Change 192.168.10 to match you own network.
By using -n 1 you are asking for only 1 packet to be sent to each computer instead of the usual 4 packets.
The above command will ping all IP Addresses on the 192.168.10.0 network and create a text document in the C:\ drive called ipaddresses.txt. This text document should only contain IP Addresses that replied to the ping request.
Although it will take quite a bit longer to complete, you can also resolve the IP Addresses to HOST names by simply adding -a to the ping command.
FOR /L %i IN (1,1,254) DO ping -a -n 1 192.168.10.%i | FIND /i "Reply">>c:\ipaddresses.txt
This is from Here