[AWS] VPC – Subnets and Routing Tables

Subnets are the logic network that you group your resources in a single local network. Communication in and out of a subnet is of critical importance, and a routing table contains the rules on how traffic can be directed.

Subnets

After creating a VPC, you can add one or more subnets in each AZ. Each subnet resides entirely within one AZ and cannot span AZs.

• A subnet is an isolated local network confined in a single AZ.
• Different application tiers are in different subnets.
• 5 IPs are reserved for a subnet (ex. /24 network – 251 IPs available)
  • 0 (first) – Network
  • 1 – Router
  • 2 – DNS
  • 3 – Future
  • X (last) – Broadcast
• Subnets must be associated with a route table.
• Subnets are by default private unless created in a default VPC.
• The CIDR block of a subnet cannot overlap the CIDR blocks of other subnets in the same VPC.

• A public subnet has a route to the internet. (It is associated with a route table that has an IGW attached.)
• A private subnet does not have a route to the internet.

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Route Tables

A route table contains a set of rules (routes) that are used to determine where traffic is directed.

• Only one route table can attached to a subnet, but, unlike an IGW, you can have multiple active route tables in a VPC.
• Every VPC has a virtual routing device called the VPC router.
• You cannot delete a route table if it has associated subnets.

• A router has a collection (route table) of rules. A route contains a destination and a target.
  • Destination: the CIDR block range of the target
  • Target: a name identifier

• Route Table is a set of rules, called routes.
  • By default, all subnets’ traffic is allowed within a VPC, which is called a local route.
  • If multiple routes apply, the most specific one is chosen (/32 is chosen before /24).
  • A default route (0.0.0.0/0 or ::/0) can be added that match any traffic not already matched.

• The default VPC already has a main route table: local route + IGW

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Public Subnets

• Subnets can be public or private.
• A subnet is public when
  1. The subnet has a public IP or EIP; You can modify auto-assign IP settings for a subnet.
  2. VPC has an Internet Gateway.
  3. The subnet has a route to send all non-local traffic to the Internet Gateway.
  4. And Network ACL and security groups allow sending and receiving traffic from the Internet.

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Notes

CIDR Visualizer: https://cidr.xyz