Hello friends, in Day 39 of my PCNSA Palo Alto Firewall Training series, I explained the real-world VPN traffic flow in a simple way using a diagram. I showed you what actually happens when a host from one private subnet wants to reach another private subnet across the internet. The key point is: the Palo Alto firewall takes the original packet (your inside traffic), encrypts the payload, and then adds a brand-new public IP header so it can travel across the internet. Internet routers only see the public source/destination IPs, not your internal subnets, and the responder firewall decrypts and forwards it to the destination PC. Same process happens for return traffic as well. Then I went deeper into NAT-T (NAT Traversal) traffic flow. If there is a NAT device in between (site1, site2, or both), IPsec needs an extra UDP header to survive port translation. During negotiation, the peers do NAT discovery/detection, and once NAT is detected, the communication shifts from UDP 500 to UDP 4500 for further messages—this is where NAT-T comes into the picture. Finally, I compared IKEv1 vs IKEv2: negotiation concept stays the same, but IKEv2 reduces messages (faster) and improves security handling, so Phase 1 and Phase 2 complete with fewer exchanges.