Overlapping CIDR
MongoDB Atlas CIDR should not overlap with your VPC/VNET CIDR when you want to perform VPC/VNET peering.
If you have an overlapping CIDR in GCP And Azure you will see errors in Atlas UI or on your cloud provider side while setting up VPC/VNET peering. However in the case of AWS, you will face issues while configuring routing .
Atlas will not allocate Private CIDR for a project when it is created. However, when you create any resources within the Atlas project ( for example cluster, Network access list, peering, private endpoint ) then by default Atlas will create VPC/VNET for your resource with 192.168.248.0/21 CIDR range.
How can I avoid overlapping CIDR?
When you are experiencing issue while creating peering due to overlapping CIDR, please consider the following steps:
1 : If you have already created a cluster in Atlas project which cannot be altered/changed and facing overlapping CIDR issues while initiating VPC/VNET peering, please consider creating a new VPC/VNET on your cloud provider’s end with CIDR which does not overlap with Atlas CIDR.
2 : If creating a new VPC/VNET is not an option, then please follow below steps:
A: Create a new Atlas project.
B: Set up a new VPC/VNET peering connection, specifying a CIDR block that does not overlap with any of your VPC/VNET CIDR.
C. Create a new Atlas cluster.
D. Leverage LiveMigration to migrate your "old" cluster to your newly created cluster.
3: If you are unable to create a new VPC/VNET on your cloud provider’s end with CIDR which does not overlap with Atlas CIDR or also not able to create a new Project in Atlas, then please consider establishing network connection via Private Endpoint.
Please note that the above option is only available for AWS and Azure. Private endpoints are subjected to additional cost. Please refer to Private Endpoints Additional Service cost for more information.
What IP address can be allocated to MongoDB Atlas Projects?
Please note that MongoDB Atlas requires your host VPC/VNET to follow the RFC-1918 standard for creating private ranges. The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets:
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
MongoDB Atlas documentation