Understanding Public IP Address and NAT

Understanding Public IP Address & NAT (Network Address Translation)


Public Address#

A public IP address is assigned to every computer that connects to the Internet where each IP is unique. Hence there cannot exist two computers with the same public IP address all over the Internet. This addressing scheme makes it possible for the computers to "find each other" online and exchange information. User has no control over the IP address (public) that is assigned to the computer. The public IP address is assigned to the computer by the Internet Service Provider as soon as the computer is connected to the Internet gateway.

Private Address#

An IP address is considered private if the IP number falls within one of the IP address ranges reserved for private networks such as a Local Area Network (LAN). The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private networks (local networks):

Class Starting IP address Ending IP address No. of hosts

A 16,777,216

B 1,048,576

C 65,536

Table 31‑1: Private IP address table

Private IP addresses are used for numbering the computers in a private network including home, school and business LANs in airports and hotels which makes it possible for the computers in the network to communicate with each other. For example, if a network A consists of 30 computers each of them can be given an IP starting from to 

Devices with private IP addresses cannot connect directly to the Internet. Likewise, computers outside the local network cannot connect directly to a device with a private IP. It is possible to interconnect two private networks with the help of a router or a similar device that supports Network Address Translation.

If the private network is connected to the Internet (through an Internet connection via ISP) then each computer will have a private IP as well as a public IP. Private IP is used for communication within the network whereas the public IP is used for communication over the Internet.

Network address translation (NAT)#

A NAT (Network Address Translation or Network Address Translator) is the virtualization of Internet Protocol (IP) addresses. NAT helps to improve security and decrease the number of IP addresses an organization needs.

A device that is configured with NAT will have at least one interface to the inside network and one to the outside network. In a typical environment, NAT is configured at the exit device between a stub domain (inside network) and the backbone. When a packet leaves the domain, NAT translates the locally significant source address into a globally unique address. When a packet enters the domain, NAT translates the globally unique destination address into a local address. If more than one exit point exists, each NAT must have the same translation table. NAT can be configured to advertise to the outside world only one address for the entire network. This ability provides additional security by effectively hiding the entire internal network behind that one address. If NAT cannot allocate an address because it has run out of addresses, it drops the packet and sends an Internet Control Message Protocol (ICMP) host unreachable packet to the destination.

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Figure 31‑1: NAT implementation

NAT is secure since it hides network from the Internet. All communications from internal private network are handled by the NAT device, which will ensure all the appropriate translations are performed and provide a flawless connection between internal devices and the Internet.

In the above figure, a simple network of 4 hosts and one router that connects this network to the Internet. All hosts in the network have a private Class C IP Address, including the router\'s private interface (, while the public interface that\'s connected to the Internet has a real IP Address ( This is the IP address the Internet sees as all internal IP addresses are hidden.

Network Setup#

Open NetSim and click on Experiments> Advanced Routing> Understanding Public IP Address and NAT (Network Address Translation) then click on the tile in the middle panel to load the example as shown in below Figure 31‑2.

Graphical user interface, text, application Description automatically
generated{width="6.416666666666667in" height="3.422222222222222in"}

Figure 31‑2: List of scenarios for the example of Understanding Public IP Address and NAT (Network Address Translation)

NetSim UI displays the configuration file corresponding to this experiment as shown below Figure 31‑3.

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Figure 31‑3: Network set up for studying the Understanding Public IP Address and NAT (Network Address Translation)


The following set of procedures were done to generate this sample:

Step 1: A network scenario is designed in NetSim GUI comprising of 6 Wired Nodes, 2 L2 Switches, and 4 Routers in the "Internetworks" Network Library.

Step 2: In the INTERFACE (ETHERNET) > NETWORK LAYER of the Wired Nodes, the IP Address and the Subnet Mask are set as per the table given below Table 31‑2.

Wired Node IP address Subnet mask







Table 31‑2: IP Address and the Subnet mask for Wired nodes

Step 3: The IP Address and the Subnet Mask in Routers are set as per the table given below Table 31‑3.

Router Interface IP address Subnet mask

Router 1 Interface_2(WAN)


Router 2 Interface_1(WAN)


Router 3 Interface_1(WAN)


Router 4 Interface_1(WAN)


Table 31‑3: IP Address and the Subnet Mask for Routers

Step 4: Right click on the Application Flow App1 CBR and select Properties or click on the Application icon present in the top ribbon/toolbar.

A CBR Application is generated from Wired Node 7 i.e. Source to Wired Node 10 i.e. Destination with Packet Size remaining 1460Bytes and Inter Arrival Time remaining 20000µs.

Additionally, the "Start Time(s)" parameter is set to 50(Figure 31‑4), while configuring the application. This time is usually set to be greater than the time taken for OSPF Convergence (i.e. Exchange of OSPF information between all the routers), and it increases as the size of the network increases.

Graphical user interface Description automatically
generated{width="5.608113517060367in" height="4.777728565179353in"}

Figure 31‑4: Application Properties Window

Step 5: Packet Trace is enabled, and hence we are able to track the route which the packets have chosen to reach the destination. Plots are enabled in NetSim GUI.

Step 6: Run the Simulation for 100 Seconds.


After simulation open Packet Trace and filter Packet ID to 1.

{width="6.5in" height="1.0715398075240594in"}

Figure 31‑5: Packet Trace

SOURCE_IP -- source node IP (Node)

DESTINATION_IP -- gateway IP/ destination IP (Router/ Node)

GATEWAY_IP -- IP of the device which is transmitting a packet (Router/ Node)

NEXT_HOP_IP -- IP of the next hop (Router/ Node)

Source node 7 ( wouldn't know how to route to the destination and hence its default gateway is Router 1 with interface IP ( So, the first line in the above screenshot specifies packet flow from Source Node 7 to L2 Switch 6 with SOURCE_IP (, DESTINATION_IP (, GATEWAY_IP ( and NEXT_HOP_IP ( Since Switch is Layer2 device there is no change in the IPs in second line. Third line specifies the packet flow from Router 1 to Router 2 with SOURCE_IP (, DESTINATION_IP ( IP of the router connected to destination. Since OSPF is running, the router is looks up the route to its destination from routing table), GATEWAY_IP ( and NEXT_HOP_IP ( and so on.