From tools like proxy and VPN that are frequently encountered in practice to the NAT operation principle of the router that is actually the most used and useful, although we do not usually think much about it, and if we need to implement such a thing at the application level, we were able to get hints on how to do it, and there were many useful parts in many ways. We were also able to briefly look at the overall security solutions of the infrastructure that commercial services run on. This lecture shows the overall forest of which areas we should focus on and supplement in the future. Those who want to work in the IT field where they need to have some network knowledge, such as basic infrastructure, server side, or communication application development that requires socket communication, to smoothly proceed with work Studying theories for qualifications is important, but if you gradually acquire the knowledge necessary for this kind of practice when you have time, it will be very helpful in many ways. Thank you for making such a great lecture. Highly recommended