What is an IP address?

The short answer#

IP stands for Internet Protocol. An IP address is a number assigned to a network connection so that other devices can send data to it. It does two jobs at once. It names the connection, so a reply knows who it is for, and it locates it inside the structure of the network, so the routers in between know which direction to forward a packet.

The usual comparison is a postal address, and it holds up better than most analogies: the address is on the envelope, not inside it, every sorting office along the way reads only enough of it to pick the next hop, and the same building can be reached from many places. The important difference is that an IP address belongs to a connection, not to a person. Move your laptop from home to a café and it gets a different address; share a flat's Wi-Fi and everyone shares one.

What an IP address looks like#

Two formats are in use. IPv4 writes 32 bits as four numbers between 0 and 255, separated by dots: 203.0.113.42. That allows 4,294,967,296 addresses, which sounded like plenty in 1981. IPv6 writes 128 bits as up to eight groups of hexadecimal, separated by colons: 2001:db8::42. The number of addresses runs to 39 digits. The IPv4 vs IPv6 guide compares them in detail, including how to read the shorthand.

The addresses in this article come from ranges reserved for documentation, so none of them belongs to a real device. Yours is on the home page, with each number picked out in its own colour.

How a request uses it#

1. Your router gave your device a private address, such as 192.168.1.14, when it joined the network. That address only means something inside your home.
2. You open a page. Your device sends a packet with that private address as the sender and the website's address as the destination.
3. Your router rewrites the sender to the public address your ISP assigned to the connection, and remembers the swap. This is network address translation.
4. Your ISP hands the packet to the next network, and that one to the next. Each network announces which addresses it can reach, and routers use that to choose a direction. The networks are identified by autonomous system numbers.
5. The website's server receives the packet. The only address it can see is your public one, so that is where it sends the reply.
6. Your router looks up the reply in its table, swaps the destination back to 192.168.1.14, and delivers it.

Before any of this, your browser had to turn the website's name into an address. That is the job of DNS, and it is a separate lookup with its own servers.

Who assigns your address#

Addresses are handed down in tiers. IANA, the Internet Assigned Numbers Authority, holds the whole pool and gives large blocks to five Regional Internet Registries. Each registry gives smaller blocks to ISPs and large organisations in its region. Your ISP then assigns one address, or on IPv6 a whole range, to your connection.

Most home connections get a dynamic address, leased for a while and changeable. Businesses often pay for a static one. The difference matters more than it sounds, and has its own guide.

What an IP address reveals, and what it does not#

Anyone who sees your public address can find out, from public data:

• The network it belongs to: the ISP or organisation, and the autonomous system that announces it.
• A rough location: usually the country, often the region, sometimes the city. This is an estimate, and it commonly points at your ISP's nearest hub rather than at you. See how accurate IP geolocation is.
• The kind of connection: home broadband, mobile, data centre or business, which is how sites guess whether a visitor is a person or a server.
• Sometimes a hostname, if the owner published a reverse DNS record.

What it does not reveal is just as important. It does not contain your name, your street address, your device, or anything you have browsed. The one party that can connect an address to a subscriber is your ISP, which knows which account held which address at which time. Everyone else is working from inference.

Common questions#