Episode 31 — Virtual Networks (VNet) and Subnets
Welcome to Episode 31, Virtual Networks (VNet) and Subnets, where we treat networking as the foundation every application stands on. A virtual network defines who can talk to whom, how, and under what conditions, so its shape quietly decides performance, security, and resilience. When this foundation is clear, developers move faster because boundaries are predictable and safe. When it is vague, teams chase ghosts—mysterious timeouts, odd latencies, and rules nobody remembers authoring. In this episode we’ll build from first principles, focusing on purpose before products, and on decisions that scale. We will keep each concept practical: what it is, why it matters, a small scenario, a misconception to avoid, and how to apply it. By the end, you will be able to sketch a clean network layout that makes change boring—in the best possible way.
A virtual network exists to provide purpose-built isolation while enabling controlled connectivity. It is your private address space in the cloud, bounded from others but able to link with them through explicit decisions like peering or gateways. That boundary matters because it defines the blast radius of mistakes and the scope of trust for sensitive data. Picture a company separating finance workloads from general applications so audits remain simple and incidents remain contained. A frequent misconception is thinking a virtual network is automatically secure; in reality, it is merely the canvas on which you apply policy. Practical application starts by naming intent—who belongs inside, who must stay out, and which shared services cross the line. Clear intent turns an abstract boundary into a dependable control.
Address spaces require planning with Classless Inter-Domain Routing, then using C I D R consistently across environments. Choose ranges that will not collide with on-premises networks or partner clouds, because overlap transforms simple routing into fragile exceptions. Think in increments you can repeat: reserve room for future subnets, test environments, and regional expansion before the first deployment. A scenario helps: you allocate a generous slash sixteen for a production environment, then carve smaller subnets for tiers; later, you still have headroom for analytics or a new product. The common misconception is choosing the smallest ranges to “save space,” only to suffocate growth six months later. Practical application means documenting chosen prefixes, owners, and purposes, and reviewing them whenever expansion is on the table.
Subnets give shape to a virtual network by aligning address boundaries to application tiers. Group front ends together, isolate application logic, and place data components behind stricter controls so rules map to real risk. This matters because segmentation narrows the impact of errors and makes troubleshooting faster: if a problem appears in the front-end subnet, you know where to start. Consider a three-tier web system where only the application tier may talk to the database tier; the subnet lines make that policy enforceable. A misconception is thinking smaller subnets are automatically more secure; clarity of intent and rules matters more than tiny blocks. Practical application: name subnets by role and environment, keep them contiguous where possible, and pair them with deliberate policies.
Service endpoints and private endpoints both keep traffic to platform services off the public internet, but they work differently. A service endpoint lets resources in a subnet reach a service over the provider’s backbone while still addressing a public endpoint. A private endpoint, by contrast, gives that service a private address in your subnet, tightening exposure even further. This distinction matters when auditors ask, “Can the service be reached from the internet?” In a payment system, private endpoints help prove containment by limiting access to private addresses only. A misconception is assuming service endpoints alone satisfy strict isolation requirements. Practically, choose private endpoints for strong isolation, and use service endpoints as a simpler step when full privatization is unnecessary.
Network Security Groups, or N S Gs, apply stateless allow and deny rules at the subnet or network interface boundary. They matter because they express least privilege in five fields—source, destination, protocol, port, and action—without needing appliances everywhere. Imagine allowing only web ports from a front-end subnet to an application subnet, while denying everything else; that single rule encodes architectural intent. A misconception is piling exceptions onto interface-level N S Gs until nobody understands the effective policy. Practical application favors broad, comprehensible rules at subnet level, with sparing interface overrides for exceptional cases. Review rules periodically, use service tags to avoid hard-coding addresses, and adopt a default-deny posture so new paths are added deliberately.
User-defined routes, often called U D R tables, let you steer packets toward specific next hops to shape traffic flow. By default, the platform supplies sensible routes, but U D Rs matter when you introduce firewalls, inspection tiers, or shared egress patterns. Picture sending all internet-bound traffic through a central firewall while keeping database traffic strictly internal; a concise route table makes that behavior reliable. A misconception is overusing granular routes for every micro-path, creating brittle black holes during change. Practically, keep tables short, comment intent, and test failover by simulating outages. When routes follow purpose rather than improvisation, troubleshooting becomes a question of design, not guesswork.
Domain Name System, or D N S, determines how names resolve inside your virtual networks, and the choice shapes how easily services find each other. Platform-provided resolution works for simple cases, while private zones let you host authoritative records that stay inside. This matters because stable names survive address changes, enabling blue-green patterns and safer rollbacks. Imagine swapping an application back end by updating an alias instead of touching code or clients. A misconception is scattering host files or hard-coded addresses, which ages poorly and blocks automation. Practically, define naming conventions that encode environment and role, centralize records, and ensure every deployment path updates D N S as a first-class step.
Hybrid name resolution links on-premises resolvers and cloud resolvers into one coherent view. Conditional forwarders, resolver proxies, or dedicated forwarder VMs allow each side to answer for its own zones while delegating the rest. This matters because users and services should not need to know where a name lives to find it. Consider a merger where two companies must interoperate quickly; hybrid forwarding avoids mass renaming while teams rationalize later. A misconception is treating D N S like an afterthought during connectivity projects, only to discover application timeouts that vanish when names work. Practically, map authoritative sources, define forward rules, monitor resolution latency, and document failure procedures so support teams can restore service under pressure.
Platform services often need private reachability from your virtual networks, and VNet integration patterns supply it. Some services attach directly into a subnet, others expose private endpoints, and some rely on outbound integration without inbound paths. The distinction matters because it decides which direction traffic flows and which controls apply. Picture a web app that must call a database privately while still receiving internet users through a gateway; integration ensures the outbound path remains private. A misconception is assuming every service supports every pattern; capabilities differ and affect design. Practically, list required services, note supported integration methods, assign address capacity for endpoints up front, and test connectivity from the exact hosts that will use it.
Avoid Internet Protocol exhaustion by planning address increments you can live with for years. Start larger than you think you need, because fragmentation and growth arrive sooner than expected. This matters when adding analytics clusters, disaster recovery, or a new region; tight ranges force disruptive renumbering. Imagine a team reserving a modest block that later cannot host private endpoints or new subnets without collisions; progress stalls for a week of address surgery. A misconception is that tiny ranges are “tidy”—they are tidy only until they are full. Practically, reserve buffer space per environment, track allocations centrally, and treat address requests like capacity planning rather than ad-hoc approvals.
Overlapping ranges and peering constraints often surface late, right when timelines tighten. Peering two virtual networks with overlapping prefixes fails or produces confusing routes, and quick workarounds rarely age well. This matters because growth frequently means connecting previously independent spaces—new regions, partners, or acquisitions. Picture discovering that development and production share the same slash twenty because of copy-paste; peering now requires painful readdressing. A misconception is believing network address translation will neatly hide every overlap; it introduces complexity and breaks assumptions. Practically, standardize non-overlapping blueprints, validate new ranges against a registry before approval, and run automated checks in pipelines to prevent collisions at source.
A checklist for secure, scalable subnet layouts keeps designs consistent without stifling flexibility. Start with clear tiers and purposeful names, size subnets with future in mind, and attach N S Gs that default to deny with explicit allowances. Add U D Rs only where traffic must traverse shared inspection points, and keep them short. Use private endpoints for sensitive platform services, align D N S with naming conventions, and wire hybrid forwarding early. Finally, verify peering and capacity plans with simple diagrams and a living registry. The misconception is that checklists slow delivery; in practice, they prevent rework and concentrate creativity where it belongs—on the application.
