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A virtual private cloud (VPC) is a logically isolated, software-defined network that you create inside a public cloud. You choose its IP address space, divide that space into subnets, define routes, and apply traffic controls so applications can communicate with each other, the internet, on-premises systems, or managed cloud services in deliberate ways. The isolation is logical rather than a promise of dedicated physical servers.
In practice, a VPC is the network boundary around cloud workloads. Its subnets determine placement, route tables determine destinations, gateways and endpoints provide connectivity, and security groups or network access controls restrict traffic. The label varies by provider: AWS calls the construct a VPC, Google Cloud calls it a VPC network, and Microsoft Azure calls its comparable construct a Virtual Network (VNet).
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What does a virtual private cloud do?
A VPC gives cloud resources a private address space and a set of rules for reaching one another and outside networks. Instead of placing every virtual machine, database, container, or managed service on one flat network, you define boundaries that match the application and its security needs.
The design is virtual: the cloud provider implements isolation with software, routing, and filtering on shared infrastructure. A VPC does not automatically encrypt every packet, make a workload private, or provide a dedicated physical data center. Those properties depend on the routes, addresses, gateways, identity controls, and encryption you configure.
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Why the network boundary matters
- Predictable addressing: A CIDR range gives workloads stable private addresses and leaves room for growth.
- Controlled exposure: Internet-facing components can be separated from databases and internal services.
- Hybrid connectivity: Routes can send traffic to an office or data center through a VPN or dedicated connection.
- Service access without public paths: Private endpoints can reach eligible cloud services without sending traffic through the public internet.
- Operational visibility: Centralized routing, filtering, and flow logging make it easier to investigate unexpected traffic.
How do VPC address ranges and subnets fit together?
You start with one or more IPv4 CIDR ranges for the VPC and add IPv6 addressing when the workload requires it. Subnets carve that larger range into smaller ranges used for placement and routing. In AWS, the VPC spans a Region, while each subnet belongs to one Availability Zone, so a highly available design normally places equivalent subnets in multiple zones.
Choosing the CIDR range
Pick a range large enough for current and future subnets, but make sure it does not overlap with networks you may connect later. Overlap complicates routing to offices, data centers, or another VPC because identical destination addresses cannot be distinguished. Reserve space for separate environments, shared services, and future zones rather than consuming every address immediately.
What a subnet actually represents
A subnet is both an IP slice and a placement domain. In AWS, resources launched into a subnet use that subnet’s route table and Availability Zone. A subnet is not inherently public or private; its effective behavior comes from the routes and addressing attached to the resources in it.
How do route tables decide where traffic goes?
A route table is a set of destination-and-target rules. When a resource sends a packet, the most specific matching route determines whether the packet stays within the VPC, goes to an internet gateway, uses a NAT gateway, reaches a private endpoint, or travels through peering, a transit gateway, or a VPN.
Typical route targets
- Local VPC route: Handles traffic between ranges that belong to the same VPC, subject to security controls.
- Internet gateway: Provides a path between the VPC and the internet when addressing and filtering also permit it.
- NAT gateway: Commonly lets resources without a direct inbound internet route make outbound connections, such as downloading updates.
- Private endpoint: Keeps access to eligible cloud services on the provider’s private network rather than requiring a public route.
- Peering or transit gateway: Connects separate cloud networks according to explicit routes.
- VPN or dedicated link: Sends traffic to an on-premises network over an encrypted tunnel or private circuit.
Route tables do not grant permission by themselves. A route can make a destination reachable, while security groups, network ACLs, host firewalls, and service policies still determine whether the connection is allowed.
What is the difference between a public and private subnet?
A public subnet has a route through an internet gateway and workloads with the addressing and security rules needed for internet communication. A private subnet lacks a direct inbound internet route; its resources can still make controlled outbound connections through NAT or reach cloud services through private endpoints.
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Load balancers, bastion hosts, and other deliberately internet-facing components are common public-subnet residents. “Public” does not mean every port is open: security groups, network ACLs, host firewalls, and application authentication must still restrict access.
Private-subnet workloads
Application backends, worker processes, and databases usually belong in private subnets. They can receive traffic from approved internal sources while remaining unreachable by unsolicited internet connections. If they need software updates or package downloads, a NAT path or private service endpoint can provide that access without placing them on a public route.
A frequent misconception
Changing a subnet’s name does nothing. A subnet becomes effectively public or private because of its route table, resource addresses, and controls. A resource with a public address can still be unusable if its routes or security rules block traffic, and a private resource can still reach the internet indirectly through NAT.
How do security groups and network ACLs work together?
In AWS, security groups filter traffic at the resource level, while network access control lists (network ACLs) filter traffic at the subnet level. Using both provides layered control rather than choosing one or the other.
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Security groups
Attach security groups to supported resources and allow only the required source, destination, protocol, and port combinations. A common pattern is to allow the load balancer to reach the application tier, the application tier to reach the database port, and no direct database access from the internet.
Network ACLs
Apply network ACL rules to a subnet when you need a coarse boundary across its resources. They can provide a broad deny or allow layer, but overly restrictive rules can break return traffic and make troubleshooting harder. Keep the intent documented and test changes against expected flows.
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Other controls still matter
Cloud identity policies, operating-system firewalls, application authentication, encryption, and managed-service policies operate above or alongside network controls. A VPC is not a substitute for those controls.
How can you design a VPC step by step?
Use this sequence before launching production workloads. The exact console labels differ by provider, but the design decisions apply across cloud platforms.
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- Inventory connections. List internet users, internal services, offices, data centers, partner networks, and managed cloud services that must communicate.
- Reserve non-overlapping address space. Select IPv4 CIDR ranges and add IPv6 ranges where needed. Compare them with every network you may connect through VPN, peering, or transit.
- Map availability and failure domains. In AWS, create equivalent subnet tiers in more than one Availability Zone when the application requires zone-level resilience.
- Define subnet tiers. Separate internet-facing components, application services, data stores, management tools, and shared services according to their exposure and routing needs.
- Assign route tables. Give each subnet the smallest set of destinations it needs. Add local, internet-gateway, NAT, endpoint, peering, transit, or VPN routes deliberately.
- Choose connectivity mechanisms. Use an internet gateway for approved public paths, NAT for controlled outbound access from private resources, endpoints for eligible private cloud-service access, and VPN, peering, or transit options for other networks.
- Write traffic rules. Start with resource-level security groups, add subnet-level network ACL rules where useful, and document permitted flows by application rather than opening broad port ranges.
- Test and observe. Verify DNS, routing, return paths, security rules, endpoint behavior, and failure between zones. Enable the provider’s available network-flow and audit logging before production traffic arrives.
- Record ownership and change rules. Keep the CIDR plan, route-table purpose, gateway dependencies, and approved ports in version-controlled documentation so later teams do not create overlapping or accidental public paths.
How does traffic flow in a typical three-tier application?
A request usually enters through an internet-facing load balancer in a public subnet. The load balancer forwards only approved application traffic to servers in private subnets; those servers query a database in another private subnet. The database has no internet route, while application servers use NAT or private endpoints only for the outbound services they require.
The route tables provide reachability between the tiers, but security groups restrict which tier can initiate each connection. If the application must call an external API, the private subnet’s default route can target NAT; replies return through the established stateful connection rather than exposing an inbound internet route to the application servers.
Is an AWS VPC the same as an Azure VNet or Google VPC?
They solve the same broad problem—an isolated cloud network—but they are not interchangeable implementations. AWS uses the term Virtual Private Cloud, Google Cloud uses VPC network, and Azure uses Virtual Network (VNet). Subnet scope, route propagation, firewall semantics, private-service access, hybrid connectivity, logging, and gateway pricing differ, so a design should be translated rather than copied.
| Provider | Network term | Comparable controls and connectivity |
|---|---|---|
| AWS | VPC | Subnets, route tables, security groups, network ACLs, internet and NAT gateways, endpoints, peering, transit gateways, and VPN. |
| Google Cloud | VPC network | Network and subnet constructs for Compute Engine, GKE, and serverless workloads; firewall, private-service, peering, and hybrid-network behavior follows Google Cloud’s model. |
| Microsoft Azure | Virtual Network (VNet) | Subnets, route controls, network security groups, private access, peering, VPN, and dedicated connectivity using Azure’s semantics. |
When moving between providers, compare network scope, whether subnets are tied to zones or regions, how routes propagate, how stateful filtering works, how IPv4 and IPv6 are assigned, which private services are eligible, and what gateways or data transfer cost. A route that is automatic in one cloud may require an explicit rule in another.
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AWS documents a default VPC in each Region for eligible accounts. It includes default subnets, an internet gateway, and a main route to the internet, which makes it convenient for experiments and simple deployments.
Custom networks are usually preferable for production because you control CIDR allocation, environment separation, internet exposure, connections to on-premises systems, and standardized security and observability. Before reusing a default network, inspect every subnet, route, gateway, and inherited security rule; convenience can conceal a broader internet path than your application needs.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What can go wrong in a VPC, and how do you fix it?
A private workload cannot reach an update server
Check that the subnet has a route to NAT or an appropriate private endpoint, that the NAT path is available in the relevant zone, and that egress security rules and DNS resolution permit the destination. A private subnet intentionally has no direct inbound internet route, so adding an internet gateway route changes its exposure and is not the first fix.
Two connected networks cannot communicate
Look for overlapping CIDR ranges first. Then verify routes exist in both directions, the peering, transit, or VPN attachment is active, and security groups, network ACLs, host firewalls, and return paths allow the flow.
A public service is unreachable
Confirm that the resource has the required public addressing, its subnet route table points to an internet gateway, and inbound rules allow only the intended ports and sources. Also check the service’s own listener, health checks, and operating-system firewall.
Traffic works in one zone but not another
Compare the subnet route tables, gateway or endpoint placement, security rules, and DNS responses across zones. Designs that rely on a single zone’s NAT or endpoint can fail when that component or zone is unavailable; duplicate critical paths where the application’s resilience target requires it.
Rules appear correct but connections still fail
Trace the complete path in both directions. A route provides a destination, not authorization, and a permissive security group cannot compensate for a missing route, blocked network ACL return rule, unresolved hostname, or application that is not listening.
What should you budget and monitor?
VPC features are often accompanied by charges for gateways, private connectivity, VPN or dedicated links, and data transfer. The exact prices depend on provider, Region, traffic volume, and product, so check the current pricing pages before choosing NAT, transit, endpoints, or cross-region designs.
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Frequently Asked Questions
Can a VPC span multiple availability zones?
Yes. In AWS, a VPC spans a Region and individual subnets are placed in Availability Zones. You create corresponding subnets in multiple zones when an application needs zone-level redundancy, then provide routes and gateways that work for each subnet.
Does a private subnet completely block internet access?
No. It blocks a direct inbound internet path, but resources can make controlled outbound connections through a NAT gateway. Private endpoints can also provide access to eligible cloud services without using a public route.
Do VPCs encrypt traffic automatically?
A VPC defines isolation and routing, not universal encryption. Use the encryption features of the service, host, VPN, or application protocol when data requires confidentiality in transit.
Why do overlapping CIDR ranges cause problems?
Routers cannot unambiguously choose between identical destination addresses on connected networks. Overlap can prevent communication through peering, transit, or VPN, so address planning should happen before those links are built.
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