What is an ISP and how do internet service providers work?

IP Addresses
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You pay a monthly bill to Comcast, AT&T, BT or Sky, and in exchange you get internet. For most people, that is where the ISP's role ends. But behind that subscription sits a massive infrastructure, commercial agreements between global operators, and a company that sees a good chunk of your digital activity pass through its network. Here is everything worth understanding about how an ISP actually works.

What an internet service provider is

An internet service provider (ISP) is a company that operates network infrastructure to connect its customers to the internet. In the United States, the residential market is dominated by Comcast (Xfinity), Charter (Spectrum), AT&T, Verizon and Cox. In the UK, BT, Sky, Virgin Media and TalkTalk hold most of the market. These companies own or lease copper lines, fibre cables, network equipment in exchanges, and negotiate interconnection deals with operators worldwide.

An ISP does far more than "give you internet". It assigns you an IP address, handles your DNS queries by default, sets the bandwidth on your line, and sometimes blocks certain content when the law requires it. It is the gateway between your home network and everything else online.

How an ISP connects you to the rest of the world

When you type an address into your browser, a precise sequence kicks off. Your device sends the request to your router, which forwards it to your ISP's nearest equipment. Depending on your technology, that is a DSLAM for DSL, an OLT for fibre, or a CMTS for cable.

From there, the request enters the ISP's backbone network: a very high-capacity infrastructure, usually fibre, that links its various points of presence domestically and internationally. If the site you want is hosted in the same country, your request may never leave national soil. If it is on the other side of the world, it travels through undersea cables and crosses several operator networks before reaching its destination, all in a few dozen milliseconds.

The key piece, usually invisible to users, is the internet exchange point (IXP). This is where ISPs physically interconnect to swap traffic. In the UK, LINX in London carries several terabits per second. In the US, Equinix runs major exchanges in New York, Chicago and Los Angeles. These interconnections let a BT customer reach a server on another network without paying transit to a third party, an arrangement known as peering.

The different types of connection

The technology that reaches your home depends heavily on where you live. The main options:

  • Fibre (FTTH, Fiber To The Home): the highest-performing connection today. The optical cable runs straight into the home. Downstream speeds commonly range from 1 to 2 Gbps, with some providers like Google Fiber offering higher. Fibre availability has expanded rapidly across both the US and UK in recent years
  • VDSL and ADSL: rely on old copper telephone lines. Speed drops with distance from the exchange. Beyond 3 km, ADSL often falls below 5 Mbps. Copper is being progressively retired across most developed markets
  • Cable (DOCSIS): inherited from coaxial TV networks. Comcast and Charter are the main cable ISPs in the US. DOCSIS 3.1 reaches 1 Gbps downstream, though upstream is usually far lower
  • Fixed 4G and 5G: a box that connects to the mobile network, useful where fibre has not arrived. Many carriers now offer "home 5G" plans that rival fibre where coverage is strong
  • Satellite: services like Starlink have transformed connectivity in remote areas, delivering 100 to 200 Mbps where no cable will ever reach

What your ISP can (and cannot) see about your activity

This is the part most people underestimate. Your ISP has real visibility into what you do online. Without specific protection, it can observe:

  • Every domain you visit, through your DNS queries if they go through its servers (the default setup)
  • The IP addresses of the servers you communicate with
  • How much data you exchange, and the timing and frequency of your connections

What it cannot read is the content of the pages you view, as long as they use HTTPS, which covers nearly every site in 2026. The content is encrypted end to end: your ISP knows you visited a site, but not what you read or wrote there.

Two tools change the privacy picture. A VPN encrypts all your traffic and routes it through a third-party server, hiding both destinations and content from your ISP. Encrypted DNS (DoH or DoT) stops your ISP from seeing your DNS queries, even if the rest of your traffic stays visible.

In the US, ISPs have been legally allowed to sell anonymised browsing data since 2017, when Congress repealed the FCC's broadband privacy rules. In the UK, the Investigatory Powers Act requires ISPs to retain connection records for 12 months and hand them to authorities on request.

The ISP hierarchy: Tier 1, 2 and 3

Not all operators play in the same league. ISPs are ranked in three tiers based on their infrastructure and independence.

Tier 1 ISPs own global infrastructure and interconnect with each other free of charge through peering. They buy transit from no one and sit at the top of the chain. The big names are AT&T, Lumen, NTT, Telia, Cogent and Tata Communications.

Tier 2 ISPs have solid national or regional infrastructure but buy transit from Tier 1 operators to reach destinations they cannot get to directly. Most consumer ISPs, including Comcast and BT, sit here: they peer extensively, but not everywhere.

Tier 3 ISPs are local or specialised operators, often serving businesses or rural areas, that buy most of their connectivity from higher-tier operators.

The IP address your ISP assigns you

As soon as you connect, your ISP assigns you a public IP address. On most residential plans this IP is dynamic: it can change when you reboot your router or after a set period. It is your identifier on the internet, visible to every server you contact.

Many ISPs offer a static IP as an add-on, usually on business plans. It typically costs between $5 and $15 a month. A static IP is essential if you host a server at home, run remotely accessible security cameras, or need reliable inbound VPN access.

Worth noting: with the IPv4 shortage, more and more ISPs use CGNAT (Carrier-Grade NAT), where multiple subscribers share a single public IP. This complicates hosting services from home and makes a static IP, or moving to IPv6, all the more useful.

How to choose an ISP

Beyond the headline price, several factors deserve attention. Availability comes first: not every ISP serves every address, especially for fibre. Check what you can actually get before committing. Then speed, but with a caveat: the advertised speed is a theoretical maximum, rarely hit in practice, particularly on copper.

Customer service quality and network reliability matter as much as raw speed. An ISP that is slightly cheaper but suffers frequent outages or has unreachable support ends up costing you in frustration. Look too at contract terms, cancellation fees, and what is genuinely included (static IP, upload speed, supplied equipment).

In many countries, ISPs can be ordered to block access to certain websites by court or regulatory decision. The block is usually implemented at the DNS level or through IP address filtering within the operator's infrastructure. This is why the same site can be reachable on one ISP and blocked on another, and why changing your DNS resolver or using a VPN often bypasses these restrictions.