VPC & Networking

VPC & networking is the boundary every AWS service sits inside (L263) — the VPC (the private network, L263), the subnets (the segments: the public for the front door L267, the private for the compute L264 and the data L268, L263), the security groups (the instance-level firewalls, L263), and the egress (the NAT gateway or the VPC endpoint for the outbound calls like Bedrock L278, L263) — the L172 baseline's network layer (L172, L263).

The one-sentence interview answer: "The VPC is the private network every AWS service sits inside (L263). It's segmented into subnets (L263): the public subnets hold the front door — the API Gateway (L267) and the load balancer — things the internet must reach (L263); the private subnets hold the compute and the data — the EC2 (L264), the RDS (L268), the ECS tasks (L271) — things the internet must not reach (L263). The firewalls are layered: the security groups (L263) — instance-level, allow-only rules — and the NACLs (L263) — subnet-level, stateless rules. The egress: the private resources reach the internet — the model calls to Bedrock (L278) — through the NAT gateway (L263) or, better, a VPC endpoint that never leaves the AWS network (L263). The AI shape: the front door public, the data private, the model calls through the endpoint (L263) — the L260 backend, networked (L263)."

Think of the VPC as the office building with locked floors. The building (the VPC, L263) is the private network — only the tenants inside may move around (L263). The floors (the subnets, L263) are the segments: the lobby (the public subnet, L263) is where the front desk (the gateway, L267) greets the visitors; the offices and the vault (the private subnets, L263) hold the work (the compute, L264) and the records (the data, L268) — not for visitors (L263). The doors (the security groups, L263) are the instance-level locks: each room opens only to the rooms that need it (L263). And the mail chute (the egress, L263) is the controlled way out: the offices reach the outside — the model provider (L278) — through the one chute (the NAT, L263) or a direct private tube (the VPC endpoint, L263). The building works because the lobby is public, the offices are locked, and the exit is controlled (L263).

   the building (the VPC, L263)
   ┌────────────────────────────────────────────────────────┐
   │ the lobby (the public subnet, L263) — the front desk    │
   │ (the gateway, L267)                                     │
   │ the offices (the private subnet, L263) — the compute    │
   │ (L264), the data (L268)                                 │
   │ the doors (the security groups, L263) — the locks       │
   │ the mail chute (the egress, L263) — the NAT / endpoint  │
   └────────────────────────────────────────────────────────┘

The mental model is the building: the lobby, the locked floors, the doors, and the mail chute (L263).

   the internet (L263)
        │
        ▼
   ┌────────────────── THE PUBLIC SUBNET (L263) ──────────────────────┐
   │  the front door: the API Gateway (L267) · the load balancer     │
   │  the security group: 443 from the internet, to the gateway (L263)│
   └──────────────────────────┬───────────────────────────────────────┘
                              ▼  (the internal call, L263)
   ┌────────────────── THE PRIVATE SUBNET (L263) ─────────────────────┐
   │  the compute (L264, L266) · the data (L268) · the queues (L270)  │
   │  the security group: the gateway's group may reach them (L263)   │
   └───────────────┬──────────────────────────────────┬───────────────┘
                   ▼                                  ▼
   ┌────────────────────────┐   ┌──────────────────────────────────┐
   │ the egress (L263)      │   │ the model access (L278)          │
   │ the NAT gateway (L263) │   │ the VPC endpoint — the private   │
   │ for the internet calls │   │ tube, never leaving AWS (L263)   │
   └────────────────────────┘   └──────────────────────────────────┘

The flow is the network: public subnet → private subnet → controlled egress (L263).

• The VPC (L263). The private network — with its own CIDR (the address range) and its own routing (L263). Every service sits inside one (L263).
• The subnets (L263). The segments: the public subnets — with a route to the internet gateway — hold the front door (L267); the private subnets — without it — hold the compute (L264) and the data (L268) (L263).
• The security groups (L263). The instance-level firewalls — allow-only, stateful (L263): the gateway's group allows 443 from the internet (L263); the compute's group allows the traffic from the gateway's group (L263).
• The NACLs (L263). The subnet-level firewalls — stateless, numbered rules (L263): the subnet's extra layer (L263).
• The egress (L263). The private resources reach the outside: the NAT gateway — the shared outbound address — for the internet calls (L263); the VPC endpoint — the private connection to the AWS services like Bedrock (L278) — never leaving the AWS network (L263).

• A serverless AI stack (L283). The API Gateway (L267) in the public layer, the Lambda (L266) in the private subnets (L263), the model calls (L278) through the VPC endpoint (L263).
• A containerized backend (L271). The ECS tasks (L271) in the private subnets (L263), the load balancer in the public (L263), the RDS (L268) in the private with a security group (L263).
• A RAG pipeline (L280). The ingestion (L280) reading the S3 (L265) in the private subnets (L263), the vector store (L268) with its own security group (L263).
• A regulated workload (L371). The private subnets for the data (L263), the egress through the endpoints (L263), the NACL for the compliance (L263).
• Anything AWS (L263). The network is the boundary (L263) — the L172 baseline's network layer (L263).

The through-line: the network is the boundary — the front door public, the work private, the egress controlled (L263).

Say it in four moves:

1. The VPC. "The private network every service sits inside (L263)."
2. The subnets. "The public — the front door (L267); the private — the compute and the data (L264, L268)."
3. The security groups. "The instance-level, allow-only firewalls (L263)."
4. The egress. "The NAT for the internet, the VPC endpoint for the AWS calls (L278) — never leaving the network (L263)."

• The network is the isolation (L320). The senior answer isolates the tenants (L320) with the network: the per-tenant subnets and security groups (L263) — the L320 isolation, VPC-shaped (L263).
• The security group is the review surface (L263). The senior review reads the security groups (L263): what may reach the database (L268), the compute (L264), the model endpoint (L278). The group rules are the attack surface (L263).
• The endpoint is the data path (L263). The model calls (L278) through the VPC endpoint (L263) never leave the AWS network (L263) — the L312 data path, private (L263).
• The egress is the blast-radius control (L263). The one NAT (L263), the scoped endpoints (L263) — the L314 agency, network-shaped (L263).
• The multi-AZ spread is the network's availability (L261). The subnets spread across the AZs (L261) — the L286 availability, networked (L263).

• Everything public (L263). The database (L268) in a public subnet (L263) — the L313 exposure (L263).
• The security group wide open (L263). 0.0.0.0/0 on the database's group (L263) — the L172 baseline lost (L263).
• The egress unplanned (L263). No NAT, no endpoint (L263) — the model calls (L278) fail or the traffic leaks through a proxy (L263).
• The data through the internet (L263). The model calls (L278) and the DB traffic (L268) over the internet (L263) — the endpoint is the private path (L263).
• The single-AZ subnets (L261). All the subnets in one AZ (L261) — the L286 availability lost (L263).

• Keep the data private (L263) — the compute (L264) and the data (L268) in the private subnets (L263).
• Scope the security groups (L263) — allow-only, by the group references (L263).
• Use the VPC endpoints (L263) — the model calls (L278) and the AWS services never leave the network (L263).
• Spread the subnets across the AZs (L261) — the availability (L286).
• Layer the NACL for the compliance (L263) — the subnet's extra belt (L263).

Q: Walk me through a VPC.

A: The private network (L263). The subnets: the public — the front door, the gateway (L267); the private — the compute (L264) and the data (L268) (L263). The firewalls: the security groups — instance-level, allow-only (L263); the NACLs — subnet-level (L263). And the egress: the NAT for the internet, the VPC endpoints for the AWS calls (L278) (L263).

Q: Why keep the data in the private subnets?

A: Because it's the L172 baseline (L172): the data (L268) must not be reachable from the internet (L263). The private subnets have no internet route (L263); the only path in is through the front door (L267) and its security group (L263). The exposure is the network's most common breach (L263).

Q: How does the private compute reach the model?

A: Two ways (L263). Through the NAT gateway — the outbound address — for the internet calls (L263); or through a VPC endpoint — the private connection to Bedrock (L278) — which never leaves the AWS network (L263). The endpoint is the senior answer: private, fast, and auditable (L263).

Q: What's the difference between a security group and a NACL?

A: The scale and the state (L263). The security group is instance-level, allow-only, and stateful (L263); the NACL is subnet-level, has allow and deny rules, and is stateless (L263). The security group is the precision tool (L263); the NACL is the subnet's extra belt (L263).

• What's the VPC (L263)?
• What are the subnets (L263)?
• What's a security group (L263)?
• What's the egress (L263)?
• Why the VPC endpoints (L263)?

The senior read: the right column is the data's home — unreachable from the internet (L263).

// The network for the L260 backend (L260, L263).
// THE VPC (L263) — the private network with its own CIDR (L263).
const vpc = { cidr: '10.0.0.0/16' };

// THE SUBNETS (L263) — the public and the private layers.
// The public: the front door (L267). The private: the work (L264, L268).
const publicSubnets  = ['10.0.1.0/24', '10.0.2.0/24'];   // AZ-a, AZ-b (L261)
const privateSubnets = ['10.0.11.0/24', '10.0.12.0/24']; // AZ-a, AZ-b (L261)

// THE SECURITY GROUPS (L263) — allow-only, by reference (L263).
const gatewaySg = {
  ingress: [{ from: 443, to: 443, cidr: '0.0.0.0/0' }],      // the HTTPS (L263)
  egress:  [{ from: 0, to: 65535, cidr: '10.0.0.0/16' }],    // the internal (L263)
};
const dataSg = {
  ingress: [{ from: 5432, to: 5432, group: gatewaySg }],      // the gateway only (L263)
  egress:  [],                                                // no outbound (L263)
};

// THE EGRESS (L263) — the model calls never leave the network (L278).
const bedrockEndpoint = { service: 'bedrock', private: true }; // the VPC endpoint (L263)

What the reader must SEE — the network, declared:

  vpc cidr 10.0.0.0/16      → the private network (L263)
  public / private subnets  → the layers: the door, the work (L263)
  gatewaySg: 443 in         → the front door's group (L263)
  dataSg: 5432 from gateway → the data's group — the gateway only (L263)
  bedrockEndpoint           → the private model path (L263, L278)

  The front door public, the data private, the egress controlled.

• The network is the latency's frame (L151). The region (L261), the AZs (L261), and the subnets (L263) — the traffic paths are the latency (L151).
• The endpoint is the fast path (L263). The VPC endpoint (L263) to Bedrock (L278) — no NAT, no internet hop (L263) — the model calls (L151) as fast as the network allows (L263).
• The security groups are the zero-cost layer (L263). The instance-level rules (L263) cost nothing and add no latency (L263).
• The NAT is the bottleneck (L263). The one NAT gateway (L263) — the shared outbound path (L263) — the throughput (L333) is its limit (L263).

• VPC & networking = the boundary (L263): the VPC, the subnets, the security groups, the egress.
• The VPC: the private network (L263).
• The subnets: the public for the door (L267), the private for the work (L264, L268).
• The security groups: the instance-level, allow-only firewalls (L263).
• The egress: the NAT (L263) and the VPC endpoints (L263) — the private model path (L278).

VPC & NETWORKING = the boundary every service sits inside

THE VPC (L263)
  the private network — the CIDR, the routing
  the subnets: the public (the door, L267) · the private (the work, L264, L268)

THE FIREWALLS (L263)
  the security groups — instance-level, allow-only, stateful
  the NACLs — subnet-level, stateless, numbered

THE EGRESS (L263)
  the NAT gateway — the shared outbound address
  the VPC endpoint — the private AWS path (L278), never leaving the network

THE AI SHAPE (L263)
  the front door public (L267) · the data private (L268)
  the model calls through the endpoint (L278)
  the subnets spread across the AZs (L261)

INTERVIEW, 4 MOVES
  1 vpc    "the private network (L263)"
  2 subnets "public for the door, private for the work (L263)"
  3 groups "instance-level, allow-only (L263)"
  4 egress "the NAT, the endpoints — the model path private (L263, L278)"

You now hold the network: the VPC, the subnets, the security groups, and the egress — with the data private and the model path controlled. The map has walls — and the L260 backend sits safely inside (L263).

Next: the baseline compute — EC2 (L264).