Understanding Private, Public, and Elastic IP Addresses

In the world of networking, IP addresses are essential for communication between networked devices. There are several types of IP addresses, each serving a specific purpose. This article aims to clarify the differences between private, public, and elastic IP addresses, helping you choose the right one for your network setup.

What is a Private IP Address?

A private IP address is an internal address used within a local network, such as a home or office. These IP addresses are not routable on the Internet and are defined by the Internet Assigned Numbers Authority (IANA) in RFCs. Common private IP address ranges include 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Devices with private IP addresses can communicate with each other within the local network, but they cannot be directly accessed from the public Internet. Routers discard packets with private IP addresses as source addresses when they attempt to route them to the Internet.

What is a Public IP Address?

A public IP address is routable on the Internet and is managed by organizations such as the Internet Corporation for Assigned Names and Numbers (ICANN) and regional network coordinating agencies (RIPE NCC, APNIC, LACNIC, etc.). Public IP addresses are typically assigned to Autonomous Systems (ASNs) and can be used to access services on the Internet. However, they can only be used within the defined scope of your network, such as your specific AS (Autonomous System) and the subnets assigned to you.

What is an Elastic IP Address?

An elastic IP address is a type of public IP address that is "invented" by cloud service providers like Amazon Web Services (AWS) and adopted by other cloud providers. An elastic IP address is not tied to a specific virtual machine (VM) or cloud resource and can be easily re-allocated to different resources as needed. This feature is particularly useful for applications that require high availability and failover capabilities. Even though it can be both private and public, it does not have a fixed meaning outside of the cloud provider's setup, as it is primarily designed for flexibility and custom resource management.

Public IP Address: Connectivity to Your Computer

A public IP address allows anyone to access your computer from the Internet. However, your computer on a local network uses a private IP address. For instance, if your computer's private IP address is 192.168.168.4, outside routers will discard the packets intended for this address because 192.168 addresses are reserved for private use. If you want to access this computer from the Internet, you need to use a public IP address and configure port forwarding or use a service like DynDNS to map a domain name to your public IP address.

Using a Public IP Address with a Private IP Address

An elastic IP address can be associated with a private IP address of a virtual network interface card (NIC) in a cloud environment. When you apply an elastic IP address to a virtual NIC, it allows external traffic to reach your virtual machine or instance. However, the virtual NIC must still route traffic via an Internet gateway to ensure that traffic can be properly routed to and from the Internet. This setup is crucial for ensuring that your cloud resources are accessible from the public Internet while maintaining the private network's functionalities.

Conclusion

Choosing the right IP address for your network needs is crucial for effective communication and security. Private IPs are ideal for local networks, public IPs ensure global access, and elastic IPs offer the flexibility of managing public access for cloud resources. Understanding these differences will help you make informed decisions regarding your network configuration.