---
title: "Autonomous System Number (ASN)"
description: "An ASN is a unique number assigned to a network operator, used by routing protocols like BGP to identify a network and its routing policies."
url: "https://mailmum.io/glossary/autonomous-system-number-asn/"
---

# Autonomous System Number (ASN)

An autonomous system number (ASN) identifies a network that is operated under a
single, consistent routing policy. It is the unit that the Border Gateway
Protocol (BGP) uses to route traffic between networks: every route on the
internet carries the list of ASNs it passed through.

For email, the relevant point is that the ASN is the largest unit at which a
sender can be identified. An IP address can be swapped in minutes and a domain
registered in seconds, but an ASN belongs to an organisation with a contract at
a Regional Internet Registry — which makes it the level at which
reputation is hardest to escape.

## Who assigns ASNs?

Regional Internet Registries (RIRs) assign them: RIPE NCC in Europe and the
Middle East, ARIN in North America, APNIC in Asia-Pacific, LACNIC in Latin
America and AFRINIC in Africa. The number space is 32-bit, so values run from 1
to 4294967294, with a reserved private range (64512–65534 and
4200000000–4294967294) for use inside a single organisation.

## Does every network have an ASN?

No, and this is where the common misconception lies. An ASN is only needed by a
network that speaks BGP to more than one other network — that is, one that
is multi-homed or that provides transit to others.

A company with a single upstream provider does not need one. Its addresses are
announced by that provider and therefore appear under the *provider's* ASN. This
matters directly: if you rent a server, its reputation at ASN level is your
hosting provider's reputation, not yours, and you inherit whatever the other
customers on that network are doing.

## How do I find the ASN for an IP address?

`whois` on the address returns the responsible ASN along with the range and the
abuse contact:

```sh
$ whois 192.0.2.25 | grep -iE 'origin|netname|abuse-mailbox'
origin:         AS64500
netname:        EXAMPLE-HOSTING
abuse-mailbox:  abuse@example-hoster.net
```

Cymru's DNS-based service maps an address to its ASN without a whois query,
which is what most tooling uses because it is fast enough for per-connection
lookups:

```sh
$ dig +short 25.2.0.192.origin.asn.cymru.com TXT
"64500 | 192.0.2.0/24 | DE | ripencc | 2019-04-11"
```

And a query on the ASN itself returns the operator:

```sh
$ dig +short AS64500.asn.cymru.com TXT
"64500 | DE | ripencc | 2019-04-11 | EXAMPLE-HOSTING, DE"
```

## Do ASNs have a reputation?

The number itself does not, but the network behind it accumulates one, and
several systems act on it.

Spamhaus publishes ASN-DROP, a list of autonomous systems considered to be under
the control of, or wholly rented out to, spam operations. Traffic from a listed
ASN is commonly discarded at the network edge rather than filtered at the mail
server. Reputation services more generally use the ASN as a feature: a
previously unseen IP address inside a range whose neighbours send spam starts
with a worse score than one in a clean network.

This is the mechanism behind snowshoe spam. An operator spreads low volumes
across many IP addresses and many domains so that no single address crosses a
threshold. What the addresses have in common is the ASN, which is why
correlating at that level catches campaigns that per-IP scoring does not.

## What does this mean if you run a mail server?

* **Check the neighbourhood before you buy.** A cheap IP address in a range with
  a poor ASN-level reputation costs more in deliverability than it saves in
  hosting fees. Look up the ASN, then look up what else is listed inside it.
* **Understand which abuse contact applies to you.** The `abuse-mailbox` in the
  RIR record for your address is where complaints about your traffic go. If it
  points at your provider, your provider hears about your problem before you do
  — see [abuse desk](https://mailmum.io/glossary/abuse-desk/).
* **Know that escalation moves upward.** Listings start at the IP address and
  widen to the range, and for networks that do not respond, eventually to the
  ASN. At that point mail from every customer in the network is affected. See
  [IP blacklisting](https://mailmum.io/glossary/ip-blacklisting/).
* **Do not expect renumbering to fix a reputation problem.** Moving to a new IP
  address inside the same ASN carries most of the problem with it.