What is the Difference Between Public and Private IP Addresses?

Unique Internet Protocol addresses, better known as IP addresses, are used to identify every computer connected to a network, whether it is a private network, such as in a workplace, or the public network that is the World Wide Web. The IP addressing scheme makes it possible for computers to locate each other and exchange information. A public IP address is assigned to every computer that connects to the Internet, while private addresses are used to distinguish between computers on the same local area network (LAN). It's similar to the way phone numbers work within an office; there is usually one main business number for a company (the public IP address), while each employee may have a separate extension so that calls can be routed to the appropriate individual (the private IP address).

An IP address is a series of binary numbers that provide information about the network and the host (the computer or other device). These numbers are typically written as four numbers separated by dots in the older, IP Version 4 (IPv4) address numbering that is most common. Because the number of available addresses in the IPv4 format is limited and running out, a new numbering scheme called IPv6 was devised in the 1990s. In this format, IP addresses are written as eight groups of four letters and numbers separated by colons, although groups with zero value may be omitted. Private addresses are known as "local-use" in IPv6.

Public IP addresses are those that allow any two computers to identify each other. When a person connects to the Internet, her computer is usually assigned an address from a pool that has been set aside for her Internet Service Provider (ISP) to use for its customers. When she types in a website address — like wisegeek.com — that domain name is converted into the IP address for the server that hosts the website. The server uses the computer's public IP address to know where to send the requested site page.

When several computers or devices are connected to each other, either with cables or wirelessly, they can make up a private network. Each device within this network is assigned a different IP address in order to exchange files and share resources within the network. Although addresses must be unique within the private network, different private networks could all use the same addresses; since the computers in different networks don't directly communicate, it doesn't matter if they have the same address. A device called a network router passes data back and forth among the connected computers using the private IP addresses as identifiers.

The private network, or one of the computers in the network, usually connects to the Internet through a modem. The router or firewall within the network is assigned a public IP address by the Internet Service Provider (ISP); this single public IP address identifies the entire network on the Internet. Using a built-in device called a Network Address Translator (NAT), the router acts as a gatekeeper and passes requests from individual computer users to the Internet. Returning data is delivered back to the public IP address, with the router determining which private IP address requested the information.

A public IP address can be static or dynamic. A static IP address does not change, and is used primarily for hosting websites or services on the Internet. Some gamers and people who use voice over IP (VOIP) regularly also prefer static IPs because it can make communication easier. A dynamic IP address is chosen from a pool of available addresses, and changes each time a given user connects to the Internet. Most computers have a dynamic public IP address, as it is the standard type of address that is assigned when a user uses the services of an ISP.

There are many programs available online that allow users to see their computer’s assigned public IP addresses, or sometimes, those of other users. It is also possible to see private IP addresses by using the network router's configuration dialogs. Novice computer users may want to consult a network administrator or other professional before attempting to access or change the information in a network router.

Within the range of publicly available IP addresses there are specific, excluded ranges withheld for private network use. In IPv4, the private IP ranges are as follows:

In IPv6, site-local addresses — the equivalent of IPv4 private addresses — begin with FE followed by C, D, E, or F. Another type, called link-local, does not have a comparison in IPv4 and is only used for special purposes on physical networks. These also begin with FE, followed by 8, 9, A, or B.

The Internet Assigned Numbers Authority (IANA) is responsible for overseeing the global allocation of IP numbers, among other related protocols. IANA, once an autonomous organization, now works under the oversight of the Internet Corporation for Assigned Names and Numbers (ICANN). ICANN is also the organization responsible for assigning and maintaining Internet domain names, such as those ending in .com and .org. IP addresses, domain names, and other identifiers simplify the complicated process of connecting computers with each other so they can readily exchange information.