What Is an ISP? How Internet Service Providers Actually Work

An ISP (Internet Service Provider) is the company that connects your home, business, or device to the internet. When you pay your monthly internet bill, you’re paying an ISP for access to their network, which in turn connects to the global internet through peering agreements and upstream providers. Your ISP assigns your public IP address, routes your traffic, resolves your DNS queries (unless you’ve changed your DNS settings), and has visibility into much of your online activity. They’re the gatekeeper between you and the rest of the internet.

ISP Tiers

The internet is structured in a hierarchy:

Tier 1 ISPs are the backbone of the internet. They own the fiber optic cables that span continents and oceans. They peer with each other at no cost (settlement-free peering) because they each need access to the others’ networks. Major Tier 1 providers include Lumen (CenturyLink), NTT, Cogent, GTT, Telia, and Zayo. You probably haven’t heard of most of them because they don’t sell to consumers.

Tier 2 ISPs purchase (or peer for) transit from Tier 1 providers and sell to consumers and businesses. They have regional networks but don’t have global reach on their own. Most large consumer ISPs are Tier 2: Comcast, AT&T, Vodafone, Deutsche Telekom, BT. These are the companies you pay your internet bill to.

Tier 3 ISPs are small, local providers that purchase transit from Tier 2 or Tier 1 providers and resell internet access to a limited geographic area. Often serve rural communities or niche markets.

What Your ISP Controls

Your IP address: Your ISP assigns your public IP from their allocated blocks. This determines your geolocation, your IP reputation, and which content restrictions apply to you.

DNS resolution: By default, your DNS queries go to your ISP’s resolvers. They can see every domain you look up, log it, and potentially filter or redirect responses.

Routing: Your ISP determines how your traffic reaches the rest of the internet. Their peering agreements and routing policies affect your latency and performance to different services.

Bandwidth: Your ISP controls the speed of your connection (both download and upload) based on your plan.

Network policies: Some ISPs throttle certain types of traffic (torrenting, streaming), inject content into HTTP pages, or use CGNAT that affects port forwarding and IP reputation.

ISP Privacy Concerns

Your ISP is in a uniquely powerful position to monitor your internet activity. They can see your DNS queries (every domain you visit), connection metadata (IP addresses, timestamps, data volumes), and unencrypted traffic content.

In the US, ISPs can legally sell aggregated browsing data to advertisers (since Congress rolled back FCC privacy rules in 2017). In the UK, the Investigatory Powers Act requires ISPs to retain browsing history for 12 months. The EU’s GDPR provides stronger protections, but ISPs still retain significant data for law enforcement cooperation.

To limit ISP visibility: use DNS over HTTPS (so they can’t see your DNS queries), use HTTPS everywhere (so they can’t read page content), and use a VPN (so they can’t see anything beyond the VPN connection itself).

Test It Yourself

Frequently Asked Questions