Route Tables

Route tables decide where network traffic from a subnet is sent, which is what makes a subnet public or private.

A route table is a set of rules (routes) that determine where traffic leaving a subnet goes. Every subnet is associated with exactly one route table.

How a route works

Each route maps a destination CIDR to a target:

  • 10.0.0.0/16 → local — traffic within the VPC (always present).
  • 0.0.0.0/0 → igw-xxxx — all other traffic to the internet gateway. This route is what makes a subnet public.
  • 0.0.0.0/0 → nat-xxxx — outbound-only internet for a private subnet via a NAT gateway.

AWS always chooses the most specific matching route for a packet.

Example

Example · bash
# Create a route table and add a default route to an internet gateway
aws ec2 create-route-table --vpc-id vpc-0a1b2c3d4e5f67890

aws ec2 create-route --route-table-id rtb-0123456789abcdef0 \
  --destination-cidr-block 0.0.0.0/0 \
  --gateway-id igw-0123456789abcdef0

# Associate it with the public subnet
aws ec2 associate-route-table \
  --route-table-id rtb-0123456789abcdef0 \
  --subnet-id subnet-0a1b2c3d4e5f67890

When to use it

  • A public subnet's route table has a 0.0.0.0/0 route to an internet gateway, making instances in it reachable from the internet.
  • A private subnet's route table has a 0.0.0.0/0 route to a NAT gateway so instances can reach the internet for updates without being publicly reachable.
  • A VPC peering connection is added to a route table so that two VPCs can communicate using private IP addresses.

More examples

Create Route Table and Add IGW Route

Creates a route table and adds a default route to an internet gateway, which makes associated subnets public.

Example · bash
RT=$(aws ec2 create-route-table \
  --vpc-id vpc-0abc123 \
  --query RouteTable.RouteTableId --output text)

# Add default route to internet gateway
aws ec2 create-route \
  --route-table-id $RT \
  --destination-cidr-block 0.0.0.0/0 \
  --gateway-id igw-0abc123

Associate Route Table with Subnet

Associates a route table with a subnet so traffic from that subnet follows the routes defined in the table.

Example · bash
aws ec2 associate-route-table \
  --route-table-id rtb-0abc123 \
  --subnet-id subnet-0abc123

View Routes in a Route Table

Shows all routes in a route table, letting you confirm which traffic goes to the internet gateway vs. stays local.

Example · bash
aws ec2 describe-route-tables \
  --route-table-ids rtb-0abc123 \
  --query 'RouteTables[].Routes[].{Dest:DestinationCidrBlock,Target:GatewayId,State:State}' \
  --output table

Related

aws vpcaws gateways

Discussion

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