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In this lesson, we’ll dive into how routing functions inside an Amazon Virtual Private Cloud (VPC). Each VPC includes an implicit, built-in router that directs traffic between subnets as well as between the VPC and external networks.

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The Implicit VPC Router

Every VPC router:
  • Has one interface per subnet, using the .1 address in each CIDR (for example, if a subnet’s CIDR is 192.168.1.0/24, its router IP is 192.168.1.1).
  • Routes traffic between subnets and to gateways or on-premises connections.
  • Is configured via route tables, where each table contains a set of routes (rules) defining how to forward packets.

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How Route Tables Work

When a packet leaves a subnet, the router:
  1. Examines the destination IP.
  2. Finds the matching prefix in the associated route table.
  3. Chooses the most specific route (longest prefix match).
  4. Forwards the packet to the route’s target (local, gateway, instance, etc.).
The router always applies the longest-prefix match if multiple routes overlap.
For example, given two IPv6 routes:
The image shows a "Route Tables" interface with two routes listed, each having a destination and a target labeled as "local."
  • 2001:db8:abcd::/48 → local
  • 2001:db8:abcd:0012::/64 → local
A packet destined for 2001:db8:abcd:0012::1234 matches both, but uses the /64 route because it has the longer prefix.

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Default Route Tables and Associations

By default, every route table in a VPC contains:
  • A local route for the VPC’s IPv4 CIDR (e.g., 10.16.0.0/16 → local)
  • If IPv6 is enabled, another local route for the IPv6 block
When a VPC is created, AWS provides a default route table. Subnets automatically associate with this default unless you specify another.
The image illustrates a default VPC setup with public subnets in two availability zones, each associated with default route tables.
You can group subnets—for instance, all public subnets—under a “public” route table that directs internet-bound traffic through an Internet Gateway. Private subnets can use a separate table routing through a NAT Gateway or other appliance.
ComponentDefault BehaviorCustomization
VPC RouterImplicit router with one interface per subnetN/A
Route TableContains a local route for IPv4 (and IPv6 if any)Add routes to IGW, NAT Gateway, VGW, etc.
Subnet AssociationEach subnet links to default route tableAssociate subnets to custom tables
Prefix MatchingChooses longest-prefix route firstCritical when CIDRs overlap
Route Targetslocal (default)IGW, NAT, instance, VPC peering, etc.
A subnet can only be associated with one route table at a time, although a route table may serve multiple subnets.

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VPC Routing Overview

The image is a summary of VPC routing concepts, detailing the role of routers, interfaces, route tables, and packet forwarding processes. It includes five key points, each marked with a colorful arrow and number.
  1. Each VPC has an implicit router with one interface per subnet (.1 address).
  2. Route tables define forwarding rules based on IP prefix destinations.
  3. The router selects the most specific matching route (longest prefix).
  4. Packets are forwarded to targets like local, gateways, or instances.
  5. Every table includes a local route for the VPC’s IPv4 (and, if enabled, IPv6) CIDR.

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Key Takeaways

  • Every VPC has one default route table.
  • Subnets must be associated with exactly one route table.
  • Multiple subnets can share the same route table.
  • Custom route tables enable distinct public/private routing behaviors.

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References

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