Noble Electronics is onboarding several acquired companies and must plan IP addressing across new VNets and the on-premises HQ. They will connect everything with VPN and ExpressRoute. What is the best address planning practice to avoid connectivity issues when VNets and on-premises networks are interconnected?

The best practice for IP addressing in hybrid environments is to ensure all connected networks, both on-premises and in Azure, have unique, non-overlapping IPv4 address spaces. Overlapping IP ranges cause routing conflicts and prevent direct communication between resources in different networks. While NAT can remediate overlapping addresses, it adds complexity, latency, and can break certain applications, so it should be used as an exception rather than a standard practice. Azure VNet peering does not automatically resolve IP conflicts. Using the same /16 range across all VNets will cause overlaps. Using public IPv4 addresses for private communication is unnecessary, costly, and does not inherently simplify routing or remove the need for NAT with private on-premises ranges.