Originally published in the Honolulu Star-Advertiser, February 25, 2020

A reader asks, “Sorry if this is a stupid question, but what is an IP address and why do I care?” While a cliche, it is especially true in that there is no such thing as a stupid question. The fact of the matter is that there is quite a bit of confusion and ignorance on the use of an IP address, even among information technology professionals.

Because this is a family newspaper, we’ll dispense with the juvenile jokes around the definition of an IP address. Short for internet protocol, an IP address is the unique identifier of a device on a network. This network could be a “private” network such as the Wi-Fi network in your home or business, or it could be the “public” network, namely the internet.

This is where it gets confusing. Devices on the internet have what is known as a “public” IP address. Not every device has a public IP address; there just aren’t enough to go around. Instead, a router or firewall typically acts as a proxy to translate private IP addresses into public IP addresses (and vice versa) when devices on a private network need to get on the internet.

IP addresses come in the form of xxx.xxx.xxx.xxx where xxx is a number between 0 and 255. The xxx is referred to as an octet. Why is a three-digit number called an octet? The explanation is too long and boring, and really, only computer science professors care.

Generally speaking, private IP addresses are 192.168.xxx.xxx, 10.xxx.xxx.xxx and, less commonly, 172.yy.xxx.xxx where yy is between 16 and 31. Public IP addresses are basically everything else.

On a private network, all of the IP addresses should look similar. If it starts with 192.168, then the first three octets should be the same, e.g. 192.168.1.xxx. If it starts with 172 or 10, then the first two octets need to be the same. If your IP address doesn’t fit such a pattern, then something is probably wrong. Another indication of a problem is the dreaded 169.254.xxx.xxx address. This address basically means your device can’t talk to anything else on the network.

The public IP is your unique identifier to the internet, analogous to a phone number. As mentioned above, the current IP addressing methods result in a limited number of available public IPs. (An initiative known as IPv6 is supposed to help with this, but it’s been around for several years now without really taking hold.) Basically, your internet service provider controls your public IP address(es) and assigns it to you when you buy their service.

Your public IP is required for you to get on the internet. As such, it is easily visible and cannot be blocked like caller ID. Unless you are using a VPN (subject of another column), just about everything you do on the internet can be traced back to your public IP. Advertisers, for example, track activities from your public IP and target you for specific ads. It is also used to identify folks who are up to no good on the internet, including cybercriminals and those who make incendiary or offensive posts on social media.

Can you change your public IP? Yes, but it’s not easy. Small office and home users can try simply turning off their modem, but unless you leave it off for at least 12 hours, and in some cases, up to 72 hours, your public IP address will not change. You can call the ISP and ask them to change it, which is actually quite easy on their part, but policies vary from ISP to ISP as to whether they will honor your request, especially without good reason. The only surefire way to change your public IP is to buy a new modem.