Public vs. Private IP (IPv4) | Overview.

An Overview of Public Vs. Private IPv4 addresses,

 Intro:

•         How they Public & Private IPv4 addresses work,
•         Why Public & Private IPv4 addresses exist,
•         How Public & Private IPv4 addresses fit into modern cloud networking (especially AWS/Azure/GCP VPCs).

Breadown

•        IPv4 Addressing Fundamentals,
•        Private vs. Public IPv4 – Core Difference,
•        RFC 1918 – Private IP Ranges,
•        Public IPs – Internet Routable Space,
•        NAT (Network Address Translation),
•        Cloud Networking Example (AWS),
•        Advanced Topics,
•        Visual Summary,
•       Key Takeaways.

 1. IPv4 Addressing Fundamentals

•        IPv4 addresses are 32-bit numbers, written as four octets (e.g., 192.168.1.10), each ranging from 0–255.
•        IPv4 addresses identify devices on a network and allow them to communicate.

Each address has:

• A network portion (identifies subnet)
• A host portion (identifies device within that subnet)

Example:
192.168.1.10/24 → Network: 192.168.1.0, Host Range: 192.168.1.1–192.168.1.254.

 2. Private vs. Public IPv4 – Core Difference

Aspect	Private IP	Public IP
Definition	Used inside private networks (LANs, VPCs, on-premises). Not routable over the internet.	Used to communicate over the internet. Routable globally.
Visibility	Only visible within the internal network.	Visible across the global internet.
Example Range (RFC 1918)	- 10.0.0.0 – 10.255.255.255 (10/8) - 172.16.0.0 – 172.31.255.255 (172.16/12) - 192.168.0.0 – 192.168.255.255 (192.168/16)	Any IPv4 address not in the private/reserved ranges (e.g., 8.8.8.8, 52.95.245.0).
Routability	Not routable on the public internet. Must go through NAT to reach outside.	Routable directly on the internet.
Typical Usage	Internal systems, backend servers, databases, private subnets, on-prem resources.	Web servers, public APIs, load balancers, internet gateways.
Security	Provides isolation by design.	Requires proper firewalling and security controls (e.g., Security Groups, NACLs).

 3. RFC 1918 – Private IP Ranges

Network	CIDR	Total Addresses	Common Usage
10.0.0.0 – 10.255.255.255	/8	~16.7 million	Large enterprise or VPC-level addressing
172.16.0.0 – 172.31.255.255	/12	~1 million	Medium-sized networks
192.168.0.0 – 192.168.255.255	/16	~65,536	Home and small office networks

 4. Public IPs – Internet Routable Space

•        Public IP addresses are assigned by Internet Assigned Number Authority (IANA) and distributed by Regional Internet Registries (RIRs) like ARIN, RIPE, APNIC.
•        Public IP addresses are globally unique and must be managed carefully to avoid conflicts.

Example Public IPs:

• Google DNS → 8.8.8.8
• AWS EC2 Instance (Elastic IP) → 54.213.12.34
• CloudFront Edge → 13.224.0.0/15

 5. NAT (Network Address Translation)

Since IPv4 address space is limited (~4.3 billion total addresses), NAT allows multiple private IPs to share a single public IP.

Types:

• SNAT (Source NAT) → private → public (outbound)
• DNAT (Destination NAT) → public → private (inbound)
• PAT (Port Address Translation) → many private IPs use one public IP via unique ports

Cloud Example:

• AWS NAT Gateway enables private subnet instances to reach the internet without exposing themselves.

 6. Cloud Networking Example (AWS)

VPC Example:

Subnet Type	CIDR	Example IP	Internet Access	Notes
Public Subnet	10.0.1.0/24.	EC2 = 10.0.1.10 (private) + 3.85.120.42 (public EIP)	Yes, via IGW	Used for web servers
Private Subnet	10.0.2.0/24.	EC2 = 10.0.2.5	Outbound via NAT Gateway only	Used for databases, backend apps

Flow Example:

1. Private instance (10.0.2.5) → NAT Gateway → Internet (via 54.x.x.x).
2. Public instance (10.0.1.10 + EIP 3.x.x.x) ↔ Direct internet access.

 7. Advanced Topics

•  Elastic IPs
• Static public IPs in AWS assigned to EC2, Load Balancers, or NAT Gateways.
•  PrivateLink / VPC Peering
• Enables private connectivity between VPCs or services without using public IPs.
•  Hybrid Cloud
• Private IPs used in VPN or Direct Connect linking on-prem networks to cloud VPCs.
•  IPv4 Exhaustion
• IPv4 addresses are scarce → IPv6 adoption (128-bit addresses) provides a nearly unlimited pool.

 8. Visual Summary

 

Key Takeaways

• Public IPs = Internet-facing, globally unique.
• Private IPs = Internal, non-routable outside the network.
• NAT bridges the gap between private and public addressing.
• VPC design uses a mix of both for security and functionality.
• IPv6 will eventually reduce dependence on NAT and IPv4 scarcity.