diff --git a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphImpl.java b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphImpl.java
new file mode 100644
index 0000000..88cedac
--- /dev/null
+++ b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphImpl.java
@@ -0,0 +1,109 @@
+package net.onrc.onos.ofcontroller.networkgraph;
+
+import java.net.InetAddress;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+
+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
+	protected ConcurrentMap<Long, Switch> switches;
+
+	protected ConcurrentMap<InetAddress, Set<Device>> addr2Device;
+	protected ConcurrentMap<MACAddress, Device> mac2Device;
+
+	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);
+	}
+
+	@Override
+	public Iterable<Switch> getSwitches() {
+		// TODO Check if it is safe to directly return this Object.
+		return Collections.unmodifiableCollection(switches.values());
+	}
+
+	@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<Link> getOutgoingLinksFromSwitch(Long dpid) {
+		Switch sw = getSwitch(dpid);
+		if (sw == null) {
+			return Collections.emptyList();
+		}
+		Iterable<Link> links = sw.getOutgoingLinks();
+		if (links instanceof Collection) {
+			return Collections.unmodifiableCollection((Collection<Link>) links);
+		} else {
+			List<Link> linklist = new LinkedList<>();
+			for (Link l : links) {
+				linklist.add(l);
+			}
+			return linklist;
+		}
+	}
+
+	@Override
+	public Iterable<Link> getIncomingLinksFromSwitch(Long dpid) {
+		Switch sw = getSwitch(dpid);
+		if (sw == null) {
+			return Collections.emptyList();
+		}
+		Iterable<Link> links = sw.getIncomingLinks();
+		if (links instanceof Collection) {
+			return Collections.unmodifiableCollection((Collection<Link>) links);
+		} else {
+			List<Link> linklist = new LinkedList<>();
+			for (Link l : links) {
+				linklist.add(l);
+			}
+			return linklist;
+		}
+	}
+
+
+	@Override
+	public Iterable<Device> getDeviceByIp(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);
+	}
+}