VPC-to-VPC

Connection Options Overview

VPC Peering

• Direct connection between two VPCs over AWS network backbone

• Can peer across accounts and regions

• Non-transitive (if A connects to B and C, B and C aren't automatically connected)

• Best for simple, static VPC connections

• Ideal for smaller environments with few VPC connections

Transit Gateway

• Hub-and-spoke architecture

• Supports up to 5,000 attachments

• Single transit gateway per region

• Can peer transit gateways across regions

• Highly scalable and flexible solution

• Supports hybrid connectivity (Direct Connect/VPN)

Transit Gateway Architecture

Attachment Requirements

• Requires transit gateway attachment in each Availability Zone

• One subnet minimum per AZ needed for communication

• Other subnets in same AZ can route through the attachment

• Supports cross-account sharing

Implementation Scenarios

1. Cross-Region Connectivity

   • Separate transit gateway per region

   • Transit gateways can be peered

   • Enables full cross-region VPC communication

1. Cross-Account Connectivity

   • Single transit gateway can be shared across accounts

   • Each account's VPCs connect via transit gateway attachments

   • All VPCs must be in same region for single gateway

1. Hybrid Connectivity

   • Supports Direct Connect and VPN connections

   • Can connect on-premises datacenters

   • Enables unified connectivity across cloud and on-premises

CIDR Considerations

CIDR Block Components

• IP address (4 octets, 0-255 per octet)

• Netmask (determines IP range size)

• AWS supports netmasks from /16 to /28

Key CIDR Rules

• VPCs cannot have overlapping CIDR ranges

• AWS reserves 5 IP addresses per subnet:

  • First 4 addresses

  • Last address

  • Reserved for AWS networking and future use

Netmask Reference

• /28 = 16 IP addresses

• Each decrease in netmask doubles available IPs

• /16 provides largest address range

• Must plan CIDR ranges carefully for VPC connectivity

Best Practices

1. Choose Connection Type Wisely

   • Use VPC peering for simple, static connections

   • Use transit gateway for scalable, complex networks

2. Plan CIDR Ranges

   • Avoid overlapping ranges

   • Account for future growth

   • Consider cross-region and cross-account needs

3. Consider Availability Zone Coverage

   • Ensure transit gateway attachments in each required AZ

   • Plan subnet distribution across AZs