Ruby NetAddr Package ref: http://qiita.com/inoueissei/items/f2d695087c2f61f5bb1b

file0.txt

require 'netaddr'

netaddr1 = NetAddr::CIDR.create('192.168.1.0/24')

# broadcastアドレスを算出
bc_address = netaddr1.last

# 数値化して1引く
gateway_int = NetAddr::CIDR.create(bc_address).to_i - 1
# その数値をアドレス表記に戻す
gateway = NetAddr.i_to_ip(gateway_int)

# /24部分だけを抽出
netmask = netaddr1.netmask

p bc_address
# => "192.168.1.255"

p gateway
# "192.168.1.254"

puts gateway + netmask
# => 192.168.1.254/24

file1.txt

require 'netaddr'

netaddr1 = NetAddr::CIDR.create('192.168.1.0/24')
netaddr2 = NetAddr::CIDR.create('192.168.1.1 255.255.0.0')
netaddr3 = NetAddr::CIDR.create('192.168.1.1 255.255.255.0')
netaddr4 = NetAddr::CIDR.create('224.0.0.1')

p netaddr1.bits
# => 24

p netaddr2.broadcast
# => "192.168.255.255"

p netaddr3.netmask
# => "/24"

p netaddr3.network
# => "192.168.1.0"

p netaddr3.subnet
# => ["192.168.1.0/25", "192.168.1.128/25"]

p netaddr1.arpa
# => "1.168.192.in-addr.arpa."

p netaddr1.cmp('192.168.0.0/16')
# => -1

p netaddr2.desc
# => "192.168.0.0/16"


#p netaddr1.enumerate
# => ["192.168.1.0", ..... , "192.168.1.255"]とダーッと出る

p netaddr2.ip
# => "192.168.1.1"

p netaddr2.last
# => "192.168.255.255"

p netaddr4.multicast_mac
# => "01-00-5e-00-00-01"

p netaddr1.next_ip
# => "192.168.2.0"

p netaddr1.next_subnet
# => "192.168.2.0/24"

p netaddr3.to_i
# => 3232235776

p NetAddr.i_to_ip(3232235776)
# => "192.168.1.0"

p netaddr1.to_s
# => "192.168.1.0/24"

p netaddr1.wildcard_mask
# => "255.255.255.0"

p netaddr1.hostmask_ext
# => "0.0.0.255"

p netaddr1.netmask_ext
# => "255.255.255.0"