Hello friends, welcome to my channel. In Day 37 of my PCNSA series, I explained the core concept of Site-to-Site VPN and why we actually need it in real networks. I used a simple branch-to-headquarter example (like Sales in a branch and Finance in HQ) where two private subnets (for example 192.168.10.0/24 and another private range) must communicate securely over the public internet. The main idea is: we create a VPN tunnel (a virtual private network) so two geographically separated sites (US to London, for example) can talk to each other safely. Then I covered the most important VPN terminologies that are very commonly asked in interviews: encryption, hashing, authentication, and Diffie-Hellman key exchange. I explained encryption as converting plain text into cipher text using an algorithm (like AES) and a key, so even if a man-in-the-middle captures the traffic, they can’t read it. Hashing, on the other hand, keeps the original text but generates a hash value to ensure integrity—if someone modifies the data in transit, the hash won’t match and the destination will drop it. Finally, I explained authentication using pre-shared key or certificates, and how Diffie-Hellman securely exchanges keys using public/private keys without sharing the private key on the wire.