RFC 1918 - Address Allocation for Private Internets
Private IP address ranges for internal networks
What are Private Networks?
RFC 1918 defines IP address ranges that are reserved for private networks and are not routed on the public internet. These addresses allow organizations to create internal networks without consuming public IP address space.
Private Address Ranges
Class A Private Range
Range: 10.0.0.0 - 10.255.255.255
CIDR: 10.0.0.0/8
Addresses: 16,777,216
Use Case: Large enterprise networks
Class B Private Range
Range: 172.16.0.0 - 172.31.255.255
CIDR: 172.16.0.0/12
Addresses: 1,048,576
Use Case: Medium-sized networks
Class C Private Range
Range: 192.168.0.0 - 192.168.255.255
CIDR: 192.168.0.0/16
Addresses: 65,536
Use Case: Small networks, home/office
Key Principles
- Non-Routable: Private addresses are not routed on the public internet
- Reusable: Multiple organizations can use the same private ranges
- NAT Required: Network Address Translation needed for internet access
- Internal Only: Only valid within the private network boundary
Network Address Translation (NAT)
Since private addresses cannot be routed on the internet, NAT is required:
- Outbound Translation: Private IPs translated to public IPs for internet access
- Inbound Translation: Public IPs translated back to private IPs for responses
- Port Mapping: Multiple private devices can share a single public IP
- Security Benefit: Internal network structure hidden from external networks
Private Networks in AWS
AWS extensively uses RFC 1918 private addressing:
VPC CIDR Blocks
- 10.0.0.0/16 - Common for large VPCs
- 172.31.0.0/16 - Default VPC range
- 192.168.0.0/24 - Small development VPCs
Subnet Design
- Public subnets: 10.0.1.0/24, 10.0.2.0/24
- Private subnets: 10.0.10.0/24, 10.0.20.0/24
- Database subnets: 10.0.100.0/24, 10.0.200.0/24
AWS Services Using Private Addressing
- EC2 Instances: Assigned private IPs from subnet ranges
- RDS Databases: Use private IPs for secure database access
- Lambda Functions: Can be placed in private subnets
- ELB Load Balancers: Internal load balancers use private IPs
- NAT Gateways: Enable private subnet internet access
Best Practices
Network Segmentation
Use different private ranges for different environments (dev, staging, prod)
Avoid Conflicts
Plan address ranges to prevent conflicts with VPN or partner networks
Subnet Planning
Leave room for growth when designing subnet boundaries
Documentation
Maintain clear documentation of IP address assignments
Common Use Cases
- Corporate Networks: Internal employee workstations and servers
- Data Centers: Server-to-server communication
- Cloud Infrastructure: Virtual machines and containers
- Home Networks: Personal devices and IoT equipment
- Development Environments: Test and staging systems