IP

You change an A record in production, but browsers keep reaching the old address. The authoritative server already returns the new value, yet users in different regions still see different answers. Packet captures clearly ask for www.example.com, but the first few hops do not return an IP directly.

The hard part of DNS is not record types. It is the model behind them: DNS is never a real-time global lookup table. It is a resolution system held together by hierarchical delegation, recursive resolution, and TTL-based caching.

When a new device plugs into the network, it usually does not know which IP address to use, who the gateway and DNS servers are, or whether the network is willing to hand out an address for a long time. The network, on the other hand, wants it online quickly, ideally without manual configuration, while still keeping control over address assignment, reclamation, and policy changes.

DHCP exists to handle exactly that. It looks like “automatic IP assignment,” but the real job is initial control-plane allocation: let the terminal join with almost no preconfiguration, while the network keeps the power to push parameters, reclaim addresses, and adjust policy later.