What is a private IP address?

Private IP address

A private (or local) IP address is an IP address that is reserved for internal use (for example computer, printer, smartTV connected to the router) behind a router or other Network Address Translation (NAT) device, apart from the public.

Why private IP addresses are used?

In public IP address range there is a limited supply, private IP addresses provide an entirely separate set of addresses that still allow access on a network but without taking up a public IP address space.

As for an example, let's consider a standard router on a home network. Most routers in homes and businesses across the globe, probably yours and your next door neighbor's, all have the IP address of 192.168.1.1, and assign 192.168.1.2, 192.168.1.3, ... to the various devices that connect to it (via something called DHCP).

It doesn't matter how many routers use the 192.168.1.1 address, or how many dozens or hundreds of devices inside that network share IP addresses with users of other networks, because they aren't communicating with each other directly.

Instead, the devices in a network use the router to translate their requests through the public IP address, which can communicate with other public IP addresses and eventually to other local networks.

This means all the devices (laptops, desktops, phones, tablets, etc.) that are contained within private networks around the world can use a private IP address with virtually no limitation, which can't be said for public IP addresses.

Private IP addresses also provide a way for devices that don't need contact with the internet, like file servers, printers, etc., to still communicate with the other devices on a network without being directly exposed to the public.

Reserved IP addresses

There is also a set of IP addresses called reserved IP addresses that are restricted even further. These are similar to private IP addresses in the sense that they can't be used for communicating on the greater internet, but they're even more restrictive than that.

The most famous reserved IP is 127.0.0.1 (localhost). This address is called the loopback address and is used to test the network adapter or integrated chip. No traffic addressed to 127.0.0.1 is sent over the local network or public internet.

Technically, the entire range from 127.0.0.0 to 127.255.255.255 is reserved for loopback purposes but you'll almost never see anything but 127.0.0.1 used in the real world.

Addresses in the range from 0.0.0.0 to 0.255.255.255 are also reserved but don't do anything at all. If you're even able to assign a device an IP address in this range, it would not function properly no matter where on the network it was installed.

Private IP address range

The Internet Assigned Numbers Authority (IANA) reserves the following IP address blocks for use as private IP addresses:

    10.0.0.0 to 10.255.255.255
    172.16.0.0 to 172.31.255.255
    192.168.0.0 to 192.168.255.255

The first set of IP addresses from above allow for over 16 million addresses, the second for over 1 million, and over 65,000 for the last range.
Another range of private IP addresses is 169.254.0.0 to 169.254.255.255 but is for Automatic Private IP Addressing (APIPA) use only.