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.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.

So/o 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 ServrHuman Resource 192.168.10.9/24 192.168.10.8/24 PC-1 192.168.10.7/24 Server-2 Accounting Server 192.168.11.9/24 192.168.11.8/24 192.168.11.7/24 PC-2

Explanation / Answer

When PC1 sends an IPv4 packet to remote server, border router replaces PC1's IP address (192.168.10.7) in the ip packet with its own IP address (209.165.200.226) and allocates a new port number in pace of the port number from PC1. It then sends the packet to the remote server. It maintains a table of the translation of PC1's ip address and port number to its own new port number. Future packets from PC1 are then translated using this table. The remote server upon receiving the ip packet from border router with IP address (209.165.200.226) established a connection with the border router instead of PC1. An incoming packet (meant for PC1) is translated by the border touter to the corresponding ip address and port of PC1 based on its tranalation table. The packet header is modifed accordingly by the border router, so that PC1 receives the packet, thinking that it came directly from the remote server. The border router (NAT device) acts as a bridge between PC1 and the remote server.

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