I'd have to say that the easiest way is most likely nmap -sP.

If you are using the 192.168.0.0 Network with a 255.255.255.0 subnet mask then you'd just have to type in nmap -sP 192.168.0.0/24 (then to write to a file you simple need to append > file.name to the end)

Your other option would be (assuming this is linux) to create a lil script to do it

Code:
#!/bin/bash
i=0
rm -rf results.txt
while [ $i -le 255 ]; do
     ping -b -c 1 192.168.0.$1
     i=`expr $i + 1`
done