In banking, telecom, and payments, reliability is not a nice to have. It is table stakes. The most reliable systems I have worked on reduce entire classes of bugs before the code even runs. Functional programming and Algebraic Data Types (ADTs) let you push correctness into the type system, so illegal states cannot be constructed in the first place.
What you will learn...
Home internet in the 90s felt simple. You plugged into Ethernet, got an IPv4 address, and you could expose a service directly. Today the path is layered and driven by economics. IPv4 did not end in a hard way. It became scarce and costly, see IPv4 address exhaustion. Data centers and enterprises still buy and route IPv4. Most residential users are placed behind Carrier-grade NAT (CGNAT) and many providers mix in IPv6 to keep compatibility while lowering costs.
This post maps that landscape with a practical lens. We will move from end to end addressing to provider networks that multiplex thousands of customers behind a few public IPs using NAT. CGNAT saves addresses and reduces ISP costs, but it blocks inbound connectivity, see NAT traversal. It also complicates games, VoIP, and P2P, and makes self-hosting fragile without extra tools....