Explain CIDR notation and how IP address planning works when designing a cloud network. How would you plan the IP address space for a multi-environment deployment?

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CIDR, which stands for Classless Inter-Domain Routing, is the standard notation for specifying IP address ranges in cloud networking. Understanding CIDR is essential for planning VPC networks, subnets, and ensuring connectivity between environments.

CIDR notation combines an IP address with a prefix length separated by a slash. For example, 10.0.0.0/16 means the first 16 bits of the address are fixed as the network portion, and the remaining 16 bits are available for host addresses. This gives you 65,536 possible addresses. A /24 block like 10.0.1.0/24 provides 256 addresses. A /28 provides 16 addresses. The smaller the prefix number, the larger the network.

Common CIDR blocks used in cloud networking fall within the RFC 1918 private address ranges: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Cloud providers typically allow VPC CIDR blocks between /16 and /28, though specific limits vary.

When planning IP addresses for a multi-environment deployment, start by allocating non-overlapping CIDR blocks for each environment. Overlapping ranges prevent VPC peering and VPN connections. A structured approach might look like this. The production VPC uses 10.0.0.0/16, staging uses 10.1.0.0/16, development uses 10.2.0.0/16, and the shared services VPC for tools like CI/CD and monitoring uses 10.3.0.0/16.

Within each VPC, subdivide the CIDR block into subnets across availability zones and tiers. For a production VPC with 10.0.0.0/16, you might create public subnets as 10.0.1.0/24 in AZ-a and 10.0.2.0/24 in AZ-b, private application subnets as 10.0.10.0/24 in AZ-a and 10.0.11.0/24 in AZ-b, and private database subnets as 10.0.20.0/24 in AZ-a and 10.0.21.0/24 in AZ-b.

Important planning considerations include leaving room for growth. Do not use the entire CIDR space immediately. If you allocate a /16 for production, use /24 subnets to leave room for additional subnets later. Account for reserved addresses in each subnet. AWS reserves five IP addresses per subnet for internal use, so a /24 subnet provides 251 usable addresses, not 256.

Also consider future connectivity needs. If you plan to connect your cloud VPCs to an on-premises network, ensure the cloud CIDR blocks do not overlap with your existing corporate network ranges. Document your IP address plan in a central registry to prevent conflicts as your cloud footprint grows.

A well-planned IP addressing scheme is like a well-organized filing system. It takes effort upfront but prevents painful redesigns as your infrastructure scales.