AWS Public IP vs. Private IP Addresses (IPv4) | Overview.

AWS Public IP vs. Private IP Addresses (IPv4)  - Overview.

Scope:

• Intro,
• Comparison Table (Public IP vs. Private IP addresses)
• Key Concepts,
• IPv4 Addressing Fundamentals,
• Private vs. Public IPv4 (Core Difference),
• RFC 1918 – Private IP Ranges,
• Sample Public IPs,
• Public IPs – Internet Routable Space,
• NAT (Network Address Translation),
• Types of NAT (Network Address Translation),
• Cloud Sample of NAT (Network Address Translation),
• Cloud Networking Sample in AWS (VPC Sample),
• Advanced Topics,
• Visual Architecture Summary,
• Key Takeaways.

  Intro:

• In AWS, the primary difference between Public IP and Private IP addresses is their scope of accessibility and behavior during resource lifecycles. 

Comparison Table (Public IP vs. Private IP addresses)

Feature	Private IP	Public IP
Reachability	Only within the VPC or internal network.	Accessible from the public internet.
Persistence	Permanent for the lifetime of the instance.	Dynamic; changes when an instance is stopped and restarted.
Visibility	Hidden from the outside world; more secure.	Unique and visible globally to everyone.
Cost	Generally no additional cost.	AWS charges for all public IPv4 addresses as of February 2024.
Primary Use Case	Internal communication (e.g., app servers to databases).	External access (e.g., hosting a web server or SSH access).

Key Concepts

• Subnet Types: A subnet is considered "public" if its route table includes a path to an Internet Gateway (IGW). 
• A "private" subnet has no direct route to the internet.
• A "private" subnet uses a NAT Gateway for outbound-only traffic.
• Elastic IP (EIP): If twtech require a public IP that does not change when an instance is stopped and started, it must allocate an Elastic IP from its AWS account.
• Security Best Practice: twtech Keeps sensitive resources (like databases) in private subnets with only private IPs.
• Keeps sensitive resources (like databases) in private subnets minimizes the attack surface. 
• twtech recommends AWS Site-to-Site VPN or Direct Connect (DX) for secure, private access from any on-premises network.

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)

Sample IPv4 addresses:

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.
Sample 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:
• American Registry for Internet Numbers (ARIN), 
• Réseaux IP Européens (RIPE), 
• Asia-Pacific Network Information Centre (APNIC).
•  Public IP addresses are globally unique and are managed carefully to avoid conflicts.

Sample 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 of NAT (Network Address Translation):

• 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 Sample of NAT (Network Address Translation):

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

 6. Cloud Networking Sample in AWS (VPC Sample):

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 Sample:

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  Architecture 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.