なんかめんどいので、normalだけ動作確認せずです。

ping.rb

# -*- coding: utf-8 -*-

#https://raw.github.com/tluna/ruby/master/network/ping.rb
require 'rubygems'
require 'net/ping'
require 'parallel'
require 'benchmark'
data = `ifconfig`

#すべてのNICからIPアドレス取得
networks = nil;
no = 1
networks = Array.new(0,nil)
data.each_line do |line|
  if line =~ /inet ([\d\.]+)/ then
    networks << $1
  end
end

#IPADDRをとってくる。
ipnet = Array.new()
i = 0
networks.each do |network|
  # loopback以外は取得
  if network != '127.0.0.1' then
    ipnet << network
  end
end

#ネットワークを表示
puts 'your networks'
ipnet.each do |ipaddr|
  puts i.to_s + ':' + ipaddr
end

#pingオブジェクト準備
ping = Net::Ping::External.new()

puts "Choose your network"
#cnetno = STDIN.gets
cnetno = '0'

cnet = ipnet[cnetno.to_i]

# networkadress抽出
if cnet =~ /([\d.]{3,})\d+$/ then
  netaddr = $1
end

targetaddr = Array.new()
for i in 0..255 do
  targetaddr << netaddr + i.to_s
end

Benchmark.bm do |x|
  x.report('normal') {
    targetaddr.each do |host|
      if ping.ping?(host) then
        puts host
      end
    end
  }
  x.report('thread') {
    results = Parallel.map(targetaddr, :in_threads => 10) do |host|
    #ping送信
      if ping.ping?(host) then
        puts host
      end
    end
  }
end