Uses Ruby (version 1.8) threading to run code in parallel which reduces the time taken for the script to complete.

require 'ping' #from core classes
@ip_prefix='192.168.1.'

def ping_ip(i)
  puts "Found #{@ip_prefix + i.to_s}" if Ping.pingecho(@ip_prefix+i.to_s,1)
end

p50 = Thread.new{(1..50).each{|i| ping_ip(i) };puts 'p50-finished'}
p100 = Thread.new{(51..100).each{|i| ping_ip(i) };puts 'p100-finished'}
p150 = Thread.new{(101..150).each{|i| ping_ip(i) }; puts 'p150-finished'}
p200 = Thread.new{(151..200).each{|i| ping_ip(i) }; puts 'p200-finished'}
p254 = Thread.new{(201..254).each{|i| ping_ip(i) }; puts 'p254-finished'}

output:

Found 192.168.1.2
Found 192.168.1.3
Found 192.168.1.8
Found 192.168.1.9
Found 192.168.1.10
Found 192.168.1.11
Found 192.168.1.12
Found 192.168.1.13
Found 192.168.1.15
Found 192.168.1.110
Found 192.168.1.21
Found 192.168.1.22
Found 192.168.1.140
p50-finished
p150-finished
Found 192.168.1.200
p200-finished
p100-finished
Found 192.168.1.254
p254-finished

This script took about 1 minute to complete, whereas the non-threaded script took around 4 minutes to complete.

Reference: Ruby Threads: RUBY THREADS [rubylearning.com]

*update 11:21pm*
I tried the same experiment again this time with 26 threads (9 pings each on average) and the script finished in under 15 seconds.

*update 11:39pm*
A separate thread for each ping seems to work best, since it only took a couple of seconds to finish.

(1..254).each{|i| Thread.new {puts 'Found ' + ip_prefix + i.to_s if Ping.pingecho(ip_prefix+i.to_s,10)}}