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NetSim IP Addressing for NAT/Public IP

1. The Source IP logged in Wireshark and packet trace is always the IP of the source
2. All routers in NetSim are NAT enabled
3. The IP Addresses of the end nodes are assumed to be private. 
4. The destination IP address field (can be seen when logged in Wireshark/packet trace) changes per Public IP/NAT rules. 
5. The public IP of a node is the IP address of the interface of the router to which it is connected. 

NetSim IP Forwarding

1. When forwarding packets within a LAN, Public IP is obviously not considered
2. If static routes are configured, then that is given higher preference over NAT/Public IP functions. Hence users would notice that end destination IP is logged in Wire-shark / packet trace

What is NAT?

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 the 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.

Examples Use Cases (Experiment file is attached which can be imported - NetSim v11,1)

1. Transmission inside a network
2. Transmission to an external network
3. Applying Static Route

Let us consider the following network configuration:

1. Transmission inside a network

We have configured UDP traffic between two nodes within the same network and two nodes that communicate across the network. In the case of Application 1 which involves Wired Node 1 and Wired Node 2 which are part of the same network, communication happens using the Private IP Address itself, without the need for NAT and Public IP Addresses.

The IP header of one of the UDP packets is shown below:

Wireshark Capture File: Wired Node 1

Wireshark Capture File: Wired Node 2

Packet Trace in NetSim:

2. Transmission to an external network

The flow graph for UDP traffic between Wired Node 1 and Wired Node 3, based on packet capture done at all the intermediate devices is shown below:

The IP header of one of the UDP packets is shown below:

Wireshark Capture File: WIRED NODE 1, where the destination IP is the gateway i.e 11.1.1.1

Wireshark Capture File: ROUTER 5, where the destination IP is the gateway i.e 11.2.1.2

Wireshark Capture File: ROUTER 6, where router is translating Public IP to private IP based on NAT Table. (i.e Real Destination IP: 11.3.1.2)

Wireshark Capture File: WIRED NODE 3

Packet Trace in NetSim: 

3. Applying Static Route

The IP header of one of the UDP packets is shown below:

Wireshark Capture File: WIRED NODE 1, both Source IP and Destination IP are end Node IP's.

Wireshark Capture File: ROUTER 5

Wireshark Capture File: ROUTER 6

Wireshark Capture File: WIRED NODE 3

Packet Trace in NetSim: 

Additional References: