Your company has recently been expanded and currently 40 users of your company n

Question

Your company has recently been expanded and currently 40 users of your company need simultaneous access to the internet. However company has only one public IP address. The network diagram of the company is presented below. Consider an example situations when three computers in switch-1 are communicating with remote server at the same time using one public IP. Assume PC-1 under switch-1 is using TCP port (1020) for this example. The public IP is assigned to the border router’s (NAT router) serial interface. Explain (for PC-1) how packet is sent to and received from the destination remote server by PC-1 through the steps: 1. source and destination addresses in outgoing packets from PC-1; 2. how router keeps track of incoming packet from remote server and forwards to correct sources with private IP inside the company host by using router’s entries.

0 S0/0 209.165.200.226/27 Remote Server Internet Fa0/0 192.168.10.1/24 Fa0/1 192.168.11.1/24 Switch-1 Switch-2 Server-1 Human Resource 192.168.10.9/24 192.168.10.8/24 PC-1 192.168.10.7/24 Server-2 192.168.11.9/24 192.168.11.8/24 192.168.11.7/24 PC-2 Accounting Server

Explanation / Answer

1. The packet from PC-1 will be forwarded to the fa0/0 interface of the router. The source IP will be 192.168.10.7/24 and source MAC address will be of PC-1. The destination IP address will be of the remote server. However, as the PC-1 cannot route through to the remote servers network on its own, it forwards those packets to the router. The routers IP address of interface fa0/0 192.168.10.1/24 has to be defined as the default Gateway on the PC-1. Then router checks its routing table and sends the packet through best possible route that it finds in the table.

2. Before understanding how router keeps track of incoming packets from remote server lets first see what Network address translation(NAT) so that we can understand the concept better. There are two types of IP address Private IP and Public IP. Public IP is used to access the internet a machine need to have one Public IP to access the internet. More than 1 machines cannot have same Public IP at the same time and there are billions of devices that needs internet access and due to the limited number of Public IP's we cannot give every machine it's own public IP. For that we use private IP we give machines a private ip like 192.168.1.1/24. but we know machine with private IP cannot connect to the internet to make it connect with internet we have a concept called NAT. It converts private ip to public IP and vice versa. let's understand it with the current scenario.

PC1 will send the packet it will contain source IP that is 192.168.10.7 its port no which is 1020 in our case remote servers IP address say its 12.9.1.1 and its port no say its 80 so out packet will look like this 192.168.10.7:1020 to 12.9.1.1:80 this packet will go to the router router will create a NAT forwarding table which looks like this

Now the packet will leave the router and it will look like this 209.165.200.226:2469 to 12.9.1.1:80. When the remote server will reply back now source ip will be 12.9.1.1:80 and destination IP will be 209.165.200.226:2469. once the router will receive the packet it will check its NAT forwarding table and will match public IP and its port with respective private IP and will change destination IP to 192.168.10.7:1020 and source IP will be 12.9.1.1:80. And finally PC1 will receive packet from the router.

Beacause of Port no multiple PC's can use same global IP with the help of Port no and NAT forwarding table router will determine which packet belongs to which pc as port no for all the machines are different.

Private side Public Side 192.168.10.7:1020 209.165.200.226:2469

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