Internet Protocols  «Prev 

Understanding Public and Private IP Addresses

As we continue to navigate the ever-evolving digital landscape, it's essential to understand the difference between public and private IP addresses. In this article, we'll explain what these terms mean and how they impact your online experience.

What is a Public IP Address?

A public IP address is a unique identifier that is assigned to a computing device and is accessible over the Internet. Just like a postal address is used to deliver physical mail to your home, a public IP address is necessary for data to be sent and received over the Internet.
To find your public IP address, you can visit a website that offers this service, such as Your public IP address is assigned to your router by your Internet Service Provider (ISP) and can be used to identify your location and the devices connected to your network.

What is a Private IP Address?

Unlike public IP addresses, private IP addresses are used to assign devices within a private network without exposing them to the Internet. For instance, if you have multiple devices within your home network, you may want to assign private IP addresses to each device.
In this scenario, your router is assigned the public IP address, and each device connected to your network (via wired or wireless connection) is assigned a unique private IP address. The router assigns these addresses via the Dynamic Host Configuration Protocol (DHCP).

Reserved IP Blocks for Private Use

To allow organizations to freely assign private IP addresses, the Network Information Center (InterNIC) has reserved certain address blocks for private use. These address blocks are not publicly routable and are only used within private networks. The following IP address blocks are reserved for private IP addresses:
  1. - (10/8 prefix)
  2. - (172.16/12 prefix)
  3. - (192.168/16 prefix)

By using private IP addresses, organizations can efficiently manage their internal networks while maintaining security and privacy.

More on Public IP Addresses

Public IP addresses are addresses recognized across the Internet. When you register with InterNIC, they issue you a public IP address, which ensures that the address is unique. However, IP addresses, like phone numbers for telephones, are becoming scarce. As a result, networks are using private IP addresses, which are valid only on a local intranet. It is never used on the Internet. The technique used for private addressing is called subnetting, and will be discussed in the next lesson. It is not necessary to register private IP addresses.

Private Networks behind Router

Small organizations with limited Internet gateway access can place their networks behind a router that keeps the addresses private and use the private IP addresses that are designed for this purpose. Private IP addresses are not recognized as valid on the Internet. The source address of IP packets originating on the private network is modified to conform to the public IP address. Conversely, IP packets from the outside are readdressed with the private destination address when they pass through the router from the outside into the private network. There are two difficulties with this approach:
  1. Traffic passing through the router to the outside, for exampleWeb requests, must be carefully tracked by the router, so that responses from the outside are routed to the correct private address on the inside. For a small network, this is relatively manageable. The task becomes far more challenging when the number of privately addressed computers is large and traffic through the interface is substantial. The correct routing of e-mail is particularly challenging.
  2. An organization with multiple local area networks connected with a backbone network must configure the private networks in such a way that traffic between the various private local area networks can be managed successfully. One possible solution is to use a single private IP addressing scheme for all networks attached within the backbone, with translation at the edge router. Again, the problem of translation becomes large and difficult to manage.
In summary, understanding the difference between public and private IP addresses is crucial in today's digital age. A public IP address is required for devices to communicate over the Internet, while private IP addresses are used within private networks to assign unique addresses to devices without exposing them to the Internet. By reserving specific IP address blocks for private use, organizations can effectively manage their internal networks while ensuring security and privacy. So, the next time you're setting up your home or business network, remember the importance of public and private IP addresses.