diff --git a/src/main/java/net/onrc/onos/core/topology/NetworkGraphImpl.java b/src/main/java/net/onrc/onos/core/topology/NetworkGraphImpl.java
new file mode 100644
index 0000000..902ffa6
--- /dev/null
+++ b/src/main/java/net/onrc/onos/core/topology/NetworkGraphImpl.java
@@ -0,0 +1,160 @@
+package net.onrc.onos.core.topology;
+
+import java.net.InetAddress;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+import net.floodlightcontroller.util.MACAddress;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class NetworkGraphImpl implements NetworkGraph {
+	@SuppressWarnings("unused")
+	private static final Logger log = LoggerFactory.getLogger(NetworkGraphImpl.class);
+
+	// DPID -> Switch
+	private ConcurrentMap<Long, Switch> switches;
+
+	private ConcurrentMap<InetAddress, Set<Device>> addr2Device;
+	private ConcurrentMap<MACAddress, Device> mac2Device;
+
+	private ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
+	private Lock readLock = readWriteLock.readLock();
+	// TODO use the write lock after refactor
+	private Lock writeLock = readWriteLock.writeLock();
+
+	public NetworkGraphImpl() {
+		// TODO: Does these object need to be stored in Concurrent Collection?
+		switches = new ConcurrentHashMap<>();
+		addr2Device = new ConcurrentHashMap<>();
+		mac2Device = new ConcurrentHashMap<>();
+	}
+
+	@Override
+	public Switch getSwitch(Long dpid) {
+		// TODO Check if it is safe to directly return this Object.
+		return switches.get(dpid);
+	}
+
+	protected void putSwitch(Switch sw) {
+		switches.put(sw.getDpid(), sw);
+	}
+
+	protected void removeSwitch(Long dpid) {
+		switches.remove(dpid);
+	}
+
+	@Override
+	public Iterable<Switch> getSwitches() {
+		// TODO Check if it is safe to directly return this Object.
+		return Collections.unmodifiableCollection(switches.values());
+	}
+
+	@Override
+	public Port getPort(Long dpid, Long number) {
+	    Switch sw = getSwitch(dpid);
+	    if (sw != null) {
+		return sw.getPort(number);
+	    }
+	    return null;
+	}
+
+	@Override
+	public Link getLink(Long dpid, Long number) {
+	    Port srcPort = getPort(dpid, number);
+	    if (srcPort == null)
+		return null;
+	    return srcPort.getOutgoingLink();
+	}
+
+	@Override
+	public Link getLink(Long srcDpid, Long srcNumber, Long dstDpid,
+			    Long dstNumber) {
+	    Link link = getLink(srcDpid, srcNumber);
+	    if (link == null)
+		return null;
+	    if (!link.getDstSwitch().getDpid().equals(dstDpid))
+		return null;
+	    if (!link.getDstPort().getNumber().equals(dstNumber))
+		return null;
+	    return link;
+	}
+
+	@Override
+	public Iterable<Link> getLinks() {
+		List<Link> linklist = new LinkedList<>();
+
+		for (Switch sw : switches.values()) {
+			Iterable<Link> links = sw.getOutgoingLinks();
+			for (Link l : links) {
+				linklist.add(l);
+			}
+		}
+		return linklist;
+	}
+
+	@Override
+	public Iterable<Device> getDevicesByIp(InetAddress ipAddress) {
+		Set<Device> devices = addr2Device.get(ipAddress);
+		if (devices == null) {
+			return Collections.emptySet();
+		}
+		return Collections.unmodifiableCollection(devices);
+	}
+
+	@Override
+	public Device getDeviceByMac(MACAddress address) {
+		return mac2Device.get(address);
+	}
+
+	protected void putDevice(Device device) {
+	    mac2Device.put(device.getMacAddress(), device);
+	    for (InetAddress ipAddr : device.getIpAddress()) {
+		Set<Device> devices = addr2Device.get(ipAddr);
+		if (devices == null) {
+		    devices = new HashSet<>();
+		    addr2Device.put(ipAddr, devices);
+		}
+		devices.add(device);
+	    }
+	}
+
+	protected void removeDevice(Device device) {
+	    mac2Device.remove(device.getMacAddress());
+	    for (InetAddress ipAddr : device.getIpAddress()) {
+		Set<Device> devices = addr2Device.get(ipAddr);
+		if (devices != null) {
+		    devices.remove(device);
+		    if (devices.isEmpty())
+			addr2Device.remove(ipAddr);
+		}
+	    }
+	}
+
+	@Override
+	public void acquireReadLock() {
+		readLock.lock();
+	}
+
+	@Override
+	public void releaseReadLock() {
+		readLock.unlock();
+	}
+
+	protected void acquireWriteLock() {
+		writeLock.lock();
+	}
+
+	protected void releaseWriteLock() {
+		writeLock.unlock();
+	}
+}