Network Migrations and Cutovers

VPC IP Address Considerations

When migrating to AWS, avoiding IP address overlaps between your VPC and on-premise networks is crucial. VPCs support netmask ranges from /16 to /28, accommodating between 65,000 and 16 hosts respectively.

Reserved IP Addresses

In every VPC, five IP addresses are automatically reserved:

• Network address (first address)

• Broadcast address (last address)

• Router address

• DNS address

• Future use address (reserved by AWS)

For example, in a /28 network with 16 addresses:

• Total addresses: 16

• Unusable addresses: 5 (reserved)

• Available addresses: 11

Network Connectivity Strategy

Initial VPN Setup

Most organizations begin their AWS journey with a VPN connection, which provides a secure and cost-effective starting point.

Transitioning to Direct Connect

As usage grows, organizations often migrate to Direct Connect while maintaining VPN as a backup solution. The transition can be managed smoothly using BGP routing:

1. Configure both VPN and Direct Connect with identical BGP prefixes

2. AWS automatically prefers the Direct Connect pathway

3. Configure on-premise network to prefer Direct Connect through either:

   • BGP weighting

   • Static routing

This setup ensures reliable connectivity while maintaining a fallback option through VPN.