diff --git a/src/main/java/net/onrc/onos/registry/controller/StandaloneRegistry.java b/src/main/java/net/onrc/onos/registry/controller/StandaloneRegistry.java
index 7e82bf0..640a49d 100644
--- a/src/main/java/net/onrc/onos/registry/controller/StandaloneRegistry.java
+++ b/src/main/java/net/onrc/onos/registry/controller/StandaloneRegistry.java
@@ -118,12 +118,17 @@
 	
 	private long blockTop = 0L;
 	private static final long BLOCK_SIZE = 0x1000000L;
+	
+	/**
+	 * Returns a block of IDs which are unique and unused.
+	 * Range of IDs is fixed size and is assigned incrementally as this method called.
+	 */
 	@Override
-	public IdBlock allocateUniqueIdBlock(){
+	public synchronized IdBlock allocateUniqueIdBlock(){
 		long blockHead = blockTop;
 		long blockTail = blockTop + BLOCK_SIZE;
 		
-		IdBlock block = new IdBlock(blockHead, blockTail, BLOCK_SIZE);
+		IdBlock block = new IdBlock(blockHead, blockTail - 1, BLOCK_SIZE);
 		blockTop = blockTail;
 		
 		return block;
diff --git a/src/main/java/net/onrc/onos/registry/controller/ZookeeperRegistry.java b/src/main/java/net/onrc/onos/registry/controller/ZookeeperRegistry.java
index f9fb62e..2d2083f 100644
--- a/src/main/java/net/onrc/onos/registry/controller/ZookeeperRegistry.java
+++ b/src/main/java/net/onrc/onos/registry/controller/ZookeeperRegistry.java
@@ -379,6 +379,12 @@
 		return data;
 	}
 	
+	/**
+	 * Returns a block of IDs which are unique and unused.
+	 * Range of IDs is fixed size and is assigned incrementally as this method called.
+	 * Since the range of IDs is managed by Zookeeper in distributed way, this method may block when
+	 * requests come up simultaneously.
+	 */
 	public IdBlock allocateUniqueIdBlock(){
 		try {
 			AtomicValue<Long> result = null;