Networking
Every enterprise Azure deployment sits on top of networking decisions made early: address space, topology, DNS, how traffic flows between on-premises and cloud. I have seen these decisions made carefully and I have inherited environments where they weren't. Fixing address space overlap mid-project is a multi-week exercise that touches every team. Getting it right upfront costs an afternoon.
What I cover here
Not the service catalogue. Microsoft documents what a VNet is. These pages cover the three networking problems I keep getting called about: spoke-to-spoke traffic that does not flow the way people expect, hybrid DNS resolution, and the Application Gateway decisions that are expensive to reverse.