Cross-Account Networking in AWS: Share VPC Prefix Lists Using RAM and Organizations

 

In modern cloud architectures, efficient and secure network configuration across multiple AWS accounts is essential. One common scenario is sharing Amazon Virtual Private Cloud (VPC) resources—particularly prefix lists—to standardize access controls and simplify network routing. AWS provides a powerful solution through Resource Access Manager (RAM) and AWS Organizations, enabling seamless cross-account networking.

This guide explores sharing VPC prefix lists using RAM and leveraging AWS Organizations to streamline access management and reduce manual configuration.

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 What Are Prefix Lists in AWS?

A prefix list in AWS is a set of CIDR blocks representing a group of IP addresses, typically for AWS-managed services like S3 or DynamoDB, or for your frequently used CIDRs. Prefix lists help simplify route tables, security group rules, and network ACLs by referencing the list rather than individual CIDR blocks.

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 Why Share Prefix Lists Across Accounts?

In large-scale environments where workloads are divided across multiple AWS accounts, it’s crucial to:

• Maintain consistent networking policies
  
  

• Reduce manual entry errors.
  
  

• Enable centralized management
  
  

• Improve security and visibility.
  
  

A central networking team can define and manage allowed IP ranges by sharing prefix lists, while other accounts can consume them without duplicating configuration.

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 Introduction to AWS RAM and AWS Organizations

AWS Resource Access Manager (RAM)

AWS RAM allows you to securely share AWS resources across accounts or within your AWS Organization. This helps reduce operational overhead and enforces consistent configurations.

AWS Organizations

This service enables the management of multiple AWS accounts under a single umbrella. With organizational units (OUs) and service control policies (SCPs), it simplifies governance and permissioning.

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 Step-by-Step: Sharing VPC Prefix Lists Across Accounts

Step 1: Create the Prefix List

Use the AWS CLI or Console to create a prefix list in the networking (central) account.

aws ec2 create-managed-prefix-list \

  --prefix-list-name "SharedServices" \

  --address-family IPv4 \

  --max-entries 5 \

  --entries Cidr=10.0.0.0/16,Description="Shared CIDR Block"

Step 2: Share the Prefix List with AWS RAM

Use AWS RAM to share the prefix list with your organization or specific accounts.

aws ram create-resource-share \

  --name "SharedPrefixList" \

  --resource-arns arn:aws:ec2:<region>:<account_id>:prefix-list/pl-xxxxxxx \

  --principals arn:aws:organizations::<org_id>:organization/o-xxxxxxx \

  --allow-external-principals

You can share:

• With specific AWS account IDs
  
  

• Entire AWS Organization
  
  

• Specific OUs within the Organization
  
  

Step 3: Accept the Resource Share (If Required)

In non-org-wide shares, the recipient account must accept the share via the AWS RAM console or CLI:

aws ram accept-resource-share-invitation \

  --resource-share-invitation-arn <invitation-arn>

If shared organization-wide, no manual acceptance is needed.

Step 4: Reference Prefix List in Security Groups and Route Tables

In the recipient accounts, you can now reference the shared prefix list:

aws ec2 authorize-security-group-ingress \

  --group-id sg-xxxxxxx \

  --protocol tcp \

  --port 443 \

  --source-prefix-list pl-xxxxxxx

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 Security and Governance Considerations

• Use tags and resource policies to track and limit usage.
  
  

• Apply Service Control Policies (SCPs) to restrict who can create or modify prefix lists.
  
  

• Enable logging and monitoring via AWS CloudTrail for auditing prefix list usage.
  
  

• Use RAM permission boundaries to enforce usage patterns across accounts.
  
  

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 Benefits of This Approach

• Centralized control over shared network access
  
  

• Reduced configuration drift across environments
  
  

• Improved security posture through consistent rule enforcement
  
  

• Fewer route table and security group entries, simplifying troubleshooting
  
  

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Conclusion

Sharing VPC prefix lists using AWS RAM and AWS Organizations is a best practice for modern cloud-native environments. It empowers central teams to define standardized network policies while enabling distributed teams to operate independently yet securely.

With careful setup and governance, this approach can significantly reduce operational overhead, improve security, and enhance cross-account communication within your AWS infrastructure.