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Internet Service Provider (ISP)

⏱ 6 min read

In one sentence: An Internet Service Provider (ISP) is a company that connects your home or phone to the internet, through its own network of routers and its links to other networks.

Why should I care?

In Router, you saw that your home router connects your home network to an Internet Service Provider (ISP). Without an ISP, your devices can talk to each other at home, but not to the rest of the world.

You probably pay an ISP: for internet at home, or for mobile data on your phone. Every web page you open travels through your ISP's network.

Analogy

An ISP is like a water company. Your house has its own pipes inside. The water company lays a pipe from the street to your house and links it to a much larger system of pipes. You pay for that link.

In the same way, your home network has its own devices and router. The ISP brings a line to your home and links it to its network of routers, and from there to the rest of the internet.

Where the analogy breaks down: water flows only into your house. Data flows both ways: you send packets out and receive packets back. A water company also supplies the water itself. An ISP does not make the web pages you see. It carries them. And unlike one water company, many ISPs join their networks, so your packets often cross several of them.

How it actually works

The line to your home

Your ISP reaches your home over a line. Common kinds of line are:

  • DSL (Digital Subscriber Line): data travels over copper telephone lines.
  • Cable: data travels over the same cables that bring television to your home.
  • Fiber: data travels through glass fibers as thin as a hair. It is usually faster than the other kinds.
  • Wireless: a radio link instead of a cable. It can stay at one building, or be mobile (more in Wi-Fi and Mobile Networks).
  • Satellite: useful in remote areas, where few people live.

At your end, a modem turns the data from your network into signals for the line, and back. As you saw in Router, the box at home often holds a modem, a switch and a router in one.

Your public address

As you saw in IP Address, addresses are handed out in steps. IANA gives large blocks of addresses to the regional registries. ISPs get their blocks through these registries. And IANA says: "Users are assigned IP addresses by Internet service providers (ISPs)."

So your ISP usually gives your home router a public address. With IPv4 and NAT (Network Address Translation), all the devices in your home can share that one address. Some ISPs go further: many of their customers share one public IPv4 address. Then your router does not hold the public address itself.

A network of networks

MDN describes an ISP as "a company that manages some special routers that are all linked together and can also access other ISPs' routers." Your packets go to your ISP's routers first. Those routers pass them on, often to another ISP, until they reach the network of the website you asked for.

The data that travels over networks is called traffic. The Internet Society names a few ways for networks to exchange it:

  • Transit: "One network pays another network to carry traffic to destinations across the broader Internet."
  • Peering: two separate networks exchange traffic directly with each other.
  • Internet exchange points (IXPs): places where many networks meet and peer, through shared equipment.

With an IXP, traffic between two networks in the same country can stay in that country. Without one, the Internet Society says, it may travel abroad and back.

A packet's path through two ISPsA laptop sends a packet through the home router and modem, and over a line to ISP A's routers. ISP A passes it, at an internet exchange point, to ISP B's routers, which deliver it to the website's network.

line

Laptop

Home router
+ modem

ISP A's routers

Internet
exchange point

ISP B's routers

Website's network

How it came to be

You do not need to remember these names and dates.

In 1985, the US started NSFNET, a network for research and education. Its national-scale part was called the backbone. A rule for the backbone banned uses "not in support of Research and Education." So commercial traffic (for business, to earn money) was not allowed there.

In 1988, a group called CNRI got permission to connect MCI Mail, a commercial email service, to the internet. The Computer History Museum says: "This was the start of commercial Internet services in the United States (and possibly the world)."

The NSF, which ran NSFNET, also asked NSFNET's smaller regional networks to find business customers. Together with the backbone rule, this was meant to help private, long-distance networks grow, such as PSI and UUNET. In April 1995, the NSFNET backbone lost its government funding.

🧪 Try it in your browser

You will see your public address, and the name of the network it belongs to. These steps change nothing.

  1. Open this address: https://stat.ripe.net/data/whats-my-ip/data.json. It belongs to RIPE NCC, one of the regional registries.
  2. What you should see: a short page of data. Some browsers show plain text with { } brackets and " " quote marks. Others show a tidy list. If you see a small + or arrow, click it to open the data.
  3. Near the end, find data. If you see a + or arrow next to it, click it. Inside data is ip. (Other words above it, such as whats-my-ip, are not the address.) Next to this ip is your public address: four numbers separated by dots (IPv4), or groups separated by colons (IPv6). Copy only the address, without the quote marks.
  4. Open this address, with your address in place of YOUR-ADDRESS: https://stat.ripe.net/data/prefix-overview/data.json?resource=YOUR-ADDRESS
  5. What you should see: find asns (inside data, near the end). Near it are asn, a number, and holder, a name. (A "warning" near the top may say "not announced" and "aligned". That is normal.)

What do these mean? An autonomous system is a network, such as an ISP's network, that is run with one set of rules for routing. Its number is the ASN. ARIN, another regional registry, says network operators need these numbers to exchange routing information with other ISPs.

So the holder is the network your address belongs to. At home, this is usually your ISP. On mobile data, it is usually your mobile company. The name may also look different from the brand name you know.

Common misunderstandings

  • "My ISP is the internet." No. Your ISP is one network among many. Your packets often cross the networks of several ISPs.
  • "Each of my devices gets its own public IPv4 address from my ISP." Not usually. With NAT, your home devices usually share one public IPv4 address.
  • "My ISP stores the websites I visit." Usually not. Websites are on computers in other networks. Your ISP carries the packets between you and them.

Recap

  • An ISP connects your home or phone to the internet, over a line such as DSL, cable, fiber, radio or satellite.
  • Your ISP usually gives your network its public IP address.
  • ISPs join their networks through transit, peering and internet exchange points, so the internet is a network of networks.

Sources

Written with AI · Reviewed by AI · Not yet checked by a human

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