diff --git a/src/main/java/net/onrc/onos/core/registry/IdBlock.java b/src/main/java/net/onrc/onos/core/registry/IdBlock.java
index 7184239..53048e4 100644
--- a/src/main/java/net/onrc/onos/core/registry/IdBlock.java
+++ b/src/main/java/net/onrc/onos/core/registry/IdBlock.java
@@ -1,31 +1,109 @@
 package net.onrc.onos.core.registry;
 
-public class IdBlock {
+import com.google.common.base.Objects;
+import net.onrc.onos.core.util.UnavailableIdException;
+
+import static com.google.common.base.Preconditions.checkArgument;
+
+/**
+ * A class representing an ID space. This class is not thread-safe.
+ */
+public final class IdBlock {
     private final long start;
-    private final long end;
     private final long size;
 
-    public IdBlock(long start, long end, long size) {
+    private long currentId;
+
+    /**
+     * Constructs a new ID block with the specified size and initial value.
+     *
+     * @param start initial value of the block
+     * @param size size of the block
+     * @throws IllegalArgumentException if the size is less than or equal to 0
+     */
+    public IdBlock(long start, long size) {
+        checkArgument(size > 0, "size should be more than 0, but %s", size);
+
         this.start = start;
-        this.end = end;
         this.size = size;
+
+        this.currentId = start;
     }
 
+    // TODO: consider if this method is needed or not
+    /**
+     * Returns the initial value.
+     *
+     * @return initial value
+     */
     public long getStart() {
         return start;
     }
 
+    // TODO: consider if this method is needed or not
+    /**
+     * Returns the last value.
+     *
+     * @return last value
+     */
     public long getEnd() {
-        return end;
+        return start + size - 1;
     }
 
+    /**
+     * Returns the block size.
+     *
+     * @return block size
+     */
     public long getSize() {
         return size;
     }
 
+    /**
+     * Returns the next ID in the block.
+     *
+     * @return next ID
+     * @throws UnavailableIdException if there is no available ID in the block.
+     */
+    public long getNextId() {
+        if (currentId > getEnd()) {
+            throw new UnavailableIdException(String.format(
+                    "use all IDs in allocated space (size: %d, end: %d, current: %d)",
+                    size, getEnd(), currentId
+            ));
+        }
+        long id = currentId;
+        currentId++;
+
+        return id;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+
+        IdBlock that = (IdBlock) o;
+        return Objects.equal(this.start, that.start)
+                && Objects.equal(this.size, that.size)
+                && Objects.equal(this.currentId, that.currentId);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hashCode(start, size, currentId);
+    }
+
     @Override
     public String toString() {
-        return "IdBlock [start=" + start + ", end=" + end + ", size=" + size
-                + "]";
+        return Objects.toStringHelper(getClass())
+                .add("start", start)
+                .add("size", size)
+                .add("currentId", currentId)
+                .toString();
     }
 }