If you’ve looked up an Autonomous System (AS) on IPinfo, you’ve probably seen a section with three tables: Peers, Upstreams, and Downstreams. This guide walks through what an ASN is, what each part of that page means, and how these network relationships actually work under the hood.
What Is an ASN?
The internet is made up of thousands of smaller networks stitched together. An ASN is like a company’s business registration number, except instead of registering a company, you’re registering “this is my piece of the internet, and I decide how traffic flows through it.”
The Autonomous System Number (ASN) identifies a network that controls its own block of IP addresses and makes its own decisions about how to route traffic. Internet Service Providers, universities, cloud providers, and large companies typically each have their own ASN. Think of it as a unique ID for one organization’s chunk of the internet.
Peering: Direct, Mutual, Usually Free
Imagine you and your neighbor each dig a private path between your two houses, just for visiting each other. You don’t let random strangers use that path to cut through to somewhere else. It only connects the two of you. That is essentially peering.
A peering relationship is a direct agreement between two networks to exchange traffic meant only for each other (and their own customers). It is typically settlement-free, meaning neither side pays the other, because both benefit roughly equally from the arrangement.
In AS1797’s case, its 3 peers include a national telecom authority, ICANN, and a regional research and education network. Each of these only exchanges traffic destined for AS1797 itself, or for AS1797’s own customers, nothing beyond that.
Upstream and Downstream: Paid, One-Directional Access
Think of your upstream as your internet service provider at home. You pay them, and in return they let you reach any website in the world, not just their own office. A downstream is just the “customer” side of that same deal, seen from the provider’s point of view.
This is a customer-provider relationship. The downstream network pays the upstream network for access to the entire internet, not just to that one network.
AS1797 has 1 upstream (AS6057), which is also one of its peers. Being both is possible: AS6057 provides AS1797 full internet reach as an upstream, while separately, the two also exchange traffic directly as peers. AS1797 has 0 downstreams, meaning no other network relies on it for internet access.
Why a Peer Isn’t “Upstream + Downstream”
A peer is like a next-door neighbor you visit directly. An upstream is like a taxi service that can take you literally anywhere in the city, but charges a fare. Being a “neighbor” and “having a taxi service” are two separate, unrelated things, not two halves of one thing. And yes, your neighbor could also happen to run the taxi service. That doesn’t merge the two relationships into one.
Peers, upstreams, and downstreams are three separate categories, not combinations of each other. The difference comes down to what gets advertised to each neighbor, and whether payment is involved.
| Relationship | What’s shared | Payment |
|---|---|---|
| Peer | Only each side’s own address space | None (settlement-free) |
| Upstream (provider) | Its entire routing table | Downstream pays upstream |
| Downstream (customer) | Just its own address space | Pays the upstream |
How This Actually Gets Decided: BGP
BGP is like two office managers calling each other on the phone and reading out a list: “Here’s the list of addresses I can deliver mail to.” Each manager decides what to say on each individual call, and to whom. Nobody shouts the list out to the whole city at once. That’s why a peer only hears “I can deliver to my own addresses,” while a paying customer hears “I can deliver everywhere.”
None of this is broadcast to the whole internet or looked up in a central directory. It happens through direct router-to-router conversations using the Border Gateway Protocol (BGP).
Here’s the sequence:
- Connection: Two networks connect their routers, either directly, through a shared switch at an Internet Exchange Point, or over a private link.
- BGP session: The routers open a BGP session between each other, manually configured by each network’s engineers.
- Route advertisement: Each router sends UPDATE messages listing which IP address blocks it can deliver traffic to.
- Policy decides what’s shared: Each network configures, per neighbor, exactly which routes to advertise. To a peer, only its own address space. To a downstream customer, its entire known routing table.
Why Pay for Upstream If Everyone Could Just Peer?
Peering is like knowing a handful of people personally, you can call them directly for free. But you can’t personally know everyone in the world. Paying for upstream is like subscribing to a phone company that already has a line to everyone else, so you can reach anyone, for a fee.
Peering only shares each side’s own address space. Peering with a network only lets you reach that specific network and its own customers, nothing else. It does not open a path to the rest of the internet.
Getting every network on the planet to agree to peer with you directly is not realistic:
- Peering agreements usually require roughly balanced traffic volumes between the two sides. A small network sending far more traffic than it receives (or vice versa) may not be an attractive peering partner.
- Each peering relationship takes its own negotiation, equipment, and often a physical presence at the same exchange point or data center.
- Large, well-connected networks often have little incentive to peer for free with smaller networks, since they can charge them for transit instead.
Paying for an upstream is how a network buys guaranteed access to everything it has not individually peered with. It trades a subscription fee for reach.
If One ASN Pays for Upstream, Can Its Peers Use It for Free?
If you pay for a taxi service that can take you anywhere in the city, you can’t then let your neighbor hop into that same taxi for free just because you two are friendly. The taxi company is only getting paid by you, so they only agreed to drive you.
When AS1797 pays AS6057 for upstream, AS6057 gives AS1797 its full routing table. But AS1797 does not turn around and hand that full table to its own peers. Only AS1797’s own address space gets advertised to its peers, exactly as before.
This rule is part of what’s known as valley-free routing. Traffic should not flow from a paying customer up to a provider, and then back down to another customer for free. If AS1797 forwarded AS6057’s full table to its peers, it would effectively be giving away free transit, undermining the very thing AS6057 is charging AS1797 for.
BGP itself does not prevent an ASN from reselling its upstream connectivity. The operator’s BGP export policies do.
When AS1797 buys transit from AS6057, AS6057 provides AS1797 with routes to the rest of the Internet. AS1797’s export policy normally allows those routes to be advertised to its customers, but not to its peers or providers.
This means a peer of AS1797 cannot normally use AS6057 as an indirect upstream through AS1797. If AS1797 deliberately advertised its provider-learned routes to that peer, it would effectively be providing transit, which is outside the normal settlement-free peering arrangement.
This is one of the practical mechanisms behind the valley-free routing model: the relationship between neighboring ASes determines which routes can be exported to them.