diff --git a/Data/IP.hs b/Data/IP.hs
--- a/Data/IP.hs
+++ b/Data/IP.hs
@@ -5,8 +5,8 @@
   -- * Documentation
   -- ** IP data
     IP (..)
-  , IPv4, toIPv4
-  , IPv6, toIPv6
+  , IPv4, toIPv4, fromIPv4
+  , IPv6, toIPv6, fromIPv6
   -- ** IP range data
   , IPRange (..)
   , AddrRange (addr, mask, mlen)
diff --git a/Data/IP/Addr.hs b/Data/IP/Addr.hs
--- a/Data/IP/Addr.hs
+++ b/Data/IP/Addr.hs
@@ -101,6 +101,26 @@
 
 ----------------------------------------------------------------
 --
+-- IPToInt
+--
+
+{-|
+  The 'fromIPv4' function convert 'IPv4' to a list of 'Int'.
+-}
+fromIPv4 :: IPv4 -> [Int]
+fromIPv4 (IP4 w) = map (\n -> fromEnum $ (w `shiftR` n) .&. 0xff) [0o30, 0o20, 0o10, 0o00]
+
+{-|
+  The 'toIPv6' function convert 'IPv6' to a list of 'Int'.
+-}
+fromIPv6 :: IPv6 -> [Int]
+fromIPv6 (IP6 (w1, w2, w3, w4)) = map fromEnum (concatMap split [w1,w2,w3,w4])
+  where
+    split :: Word32 -> [Word32]
+    split n = [n `shiftR` 0x10 .&. 0xffff, n .&. 0xffff]
+
+----------------------------------------------------------------
+--
 -- Read
 --
 
diff --git a/iproute.cabal b/iproute.cabal
--- a/iproute.cabal
+++ b/iproute.cabal
@@ -1,5 +1,5 @@
 Name:                   iproute
-Version:                1.2.3
+Version:                1.2.4
 Author:                 Kazu Yamamoto <kazu@iij.ad.jp>
 Maintainer:             Kazu Yamamoto <kazu@iij.ad.jp>
 License:                BSD3
diff --git a/test/Tests.hs b/test/Tests.hs
--- a/test/Tests.hs
+++ b/test/Tests.hs
@@ -40,6 +40,8 @@
              , testProperty "Delete insert IPv6" prop_delete_insert_ipv6
              , testProperty "Delete lookup IPv4" prop_delete_lookup_ipv4
              , testProperty "Delete lookup IPv6" prop_delete_lookup_ipv6
+             , testProperty "Convert fromto [Int] IPv4" prop_convert_ipv4
+             , testProperty "Convert fromto [Int] IPv6" prop_convert_ipv6
              ]
         , testGroup "Test case for routing table" [
                testCase "fromto IPv4" test_fromto_ipv4
@@ -81,18 +83,28 @@
 --
 
 instance Arbitrary (AddrRange IPv4) where
-    arbitrary = arbitraryIP toIPv4 255 4 32
+    arbitrary = arbitraryIP arbitrary 32
 
 instance Arbitrary (AddrRange IPv6) where
-    arbitrary = arbitraryIP toIPv6 65535 8 128
+    arbitrary = arbitraryIP arbitrary 128
 
-arbitraryIP :: Routable a => ([Int] -> a) -> Int -> Int -> Int -> Gen (AddrRange a)
-arbitraryIP func width adrlen msklen = do
-  a <- replicateM adrlen (choose (0,width))
-  let adr = func a
-  len <- choose (0,msklen)
-  return $ makeAddrRange adr len
+instance Arbitrary IPv4 where
+    arbitrary = arbitraryAdr toIPv4 255 4
 
+instance Arbitrary IPv6 where
+    arbitrary = arbitraryAdr toIPv6 65535 8
+
+arbitraryAdr :: Routable a => ([Int] -> a) -> Int -> Int -> Gen a
+arbitraryAdr func width adrlen = do
+    a <- replicateM adrlen (choose (0, width))
+    return $ func a
+
+arbitraryIP :: Routable a => Gen a -> Int -> Gen (AddrRange a)
+arbitraryIP adrGen msklen = do
+    adr <- adrGen
+    len <- choose (0,msklen)
+    return $ makeAddrRange adr len
+
 ----------------------------------------------------------------
 --
 -- Properties
@@ -233,6 +245,12 @@
     rs = zip xs xs
     t = fromList rs
     k = head xs
+
+prop_convert_ipv4 :: IPv4 -> Property
+prop_convert_ipv4 ip = True ==> (toIPv4 . fromIPv4) ip == ip
+
+prop_convert_ipv6 :: IPv6 -> Property
+prop_convert_ipv6 ip = True ==> (toIPv6 . fromIPv6) ip == ip
 
 ----------------------------------------------------------------