AWS – Recommended CIDR for Creating VPC

amazon-vpccidr

I have been creating AWS VPCs and I am wondering if there is a recommended CIDR value when creating VPCs. What are the factors that I must consider when choosing a CIDR and does the CIDR value affect the performance of the network?

Best Answer

I would recommend the following considerations:

If you creating an IPSEC connection between your corporate LAN and your VPC, use a CIDR that is different than that on your corporate LAN. This will prevent routing overlaps and create an identity distinction for reference.

For very large networks, use at least different 16-bit masks in different regions eg

eu-west-1 10.1.0.0/16
us-east-1 10.2.0.0/16
us-west-1 10.3.0.0/16

For smaller networks, use a 24-bit mask in different regions eg

eu-west-1 10.0.1.0/24
us-east-1 10.0.2.0/24
us-west-1 10.0.3.0/24

Consider making a distinction between private and public subnets, eg

private 10.0.1.0/24 (3rd byte < 129)
public 10.0.129.0/24 (3rd byte > 128)

Don't over-allocate address space to subnets, eg

eu-west-1 10.0.1.0/26
eu-west-1 10.0.1.64/26
eu-west-1 10.0.1.128/26
eu-west-1 10.0.1.192/26

(62 hosts per subnet)

Don't under-allocate either. If you use a load of Elastic Load Balancers, remember that they will also consume available ip addresses on your subnets. This is a particularly true if you use ElasticBeanstalk.

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