IP addressing and CIDR (Classless Inter-Domain Routing) blocks are fundamental concepts for designing AWS network architectures. An IP address is a unique numerical identifier assigned to each device in a network, enabling communication between resources. In AWS, you work primarily with IPv4 addres…IP addressing and CIDR (Classless Inter-Domain Routing) blocks are fundamental concepts for designing AWS network architectures. An IP address is a unique numerical identifier assigned to each device in a network, enabling communication between resources. In AWS, you work primarily with IPv4 addresses (32-bit) and increasingly with IPv6 addresses (128-bit).
CIDR notation provides a method for allocating IP addresses and defining network boundaries. It consists of an IP address followed by a forward slash and a number (e.g., 10.0.0.0/16). The number after the slash represents the network prefix length, indicating how many bits are fixed for the network portion. The remaining bits determine available host addresses.
For AWS VPC design, understanding CIDR is essential. When creating a VPC, you must specify a CIDR block between /16 (65,536 addresses) and /28 (16 addresses). Common private IP ranges include 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.
Key considerations for organizational complexity include:
1. **Non-overlapping ranges**: When connecting multiple VPCs through peering or Transit Gateway, CIDR blocks must not overlap to enable proper routing.
2. **Subnet planning**: Divide your VPC CIDR into smaller subnets across Availability Zones, reserving space for future growth.
3. **Secondary CIDRs**: AWS allows adding secondary CIDR blocks to existing VPCs, providing flexibility for expansion.
4. **IP Address Management (IPAM)**: AWS VPC IPAM helps organizations plan, track, and monitor IP addresses across accounts and regions.
5. **Reserved addresses**: AWS reserves five IP addresses per subnet for networking purposes.
For multi-account architectures, establish a centralized IP address allocation strategy to prevent conflicts. Consider using larger CIDR blocks initially and implementing a hierarchical addressing scheme that aligns with your organizational structure and anticipated growth patterns.
IP Addressing and CIDR Blocks - Complete Guide
Why IP Addressing and CIDR Blocks Matter
Understanding IP addressing and CIDR (Classless Inter-Domain Routing) blocks is fundamental to designing AWS network architectures. This knowledge is essential for creating VPCs, subnets, and establishing connectivity between on-premises networks and AWS. Poor IP address planning can lead to routing conflicts, wasted address space, and inability to scale your infrastructure.
What is CIDR?
CIDR is a method for allocating IP addresses and routing IP packets. It replaced the old classful addressing system (Class A, B, C) with a more flexible approach. CIDR notation combines an IP address with a suffix indicating the number of bits in the network prefix.
Key Formula: Number of available addresses = 2^(32 - prefix_length)
AWS VPC CIDR Considerations
- VPC CIDR range: /16 (largest) to /28 (smallest) - AWS reserves 5 IP addresses in each subnet (first 4 and last 1) - Secondary CIDR blocks can be added to VPCs - CIDR blocks must not overlap with existing VPCs when peering
- Plan for growth; choose larger CIDR blocks when possible - Use /24 or larger for most subnets - Consider high availability across multiple Availability Zones - Leave room for additional subnets in your VPC design
Exam Tips: Answering Questions on IP Addressing and CIDR Blocks
1. Memorize Common CIDR Calculations: Know that /24 = 256 IPs, /25 = 128 IPs, /26 = 64 IPs, /27 = 32 IPs, /28 = 16 IPs. For AWS subnets, subtract 5 for usable addresses.
2. Watch for Overlapping CIDR Scenarios: Questions often test whether CIDR blocks can be peered or connected. Overlapping ranges cannot be peered. Example: 10.0.0.0/16 overlaps with 10.0.1.0/24.
3. Understand Non-Overlapping Requirements: VPC peering, Transit Gateway attachments, and VPN connections all require non-overlapping CIDR blocks. When a question mentions connectivity issues, check for overlaps first.
4. Calculate Quickly Using Powers of 2: Each decrease in prefix length doubles the addresses. /24 to /23 doubles from 256 to 512.
5. Remember AWS-Reserved Addresses: In a /28 subnet (16 IPs), only 11 are usable. This is commonly tested when sizing subnets for specific workloads.
6. Secondary CIDR Blocks: If a question mentions running out of IP addresses in a VPC, adding a secondary CIDR block is often the solution.
7. On-Premises Integration: When connecting to on-premises networks, ensure the VPC CIDR does not conflict with existing corporate IP ranges. Questions may present scenarios where you must choose appropriate non-conflicting ranges.
8. Largest and Smallest Boundaries: Remember VPC limits: /16 maximum size, /28 minimum size. Subnet must be within VPC CIDR range.