The data

Where the snapshots come from

CAIDA derives a prefix-to-origin-AS mapping from RouteViews BGP tables once a day and publishes it as the Routeviews Prefix to AS mappings Dataset (pfx2as):

Family	Directory on publicdata.caida.org	File names
IPv4	datasets/routing/routeviews-prefix2as/YYYY/MM/	routeviews-rv2-YYYYMMDD-HHMM.pfx2as.gz
IPv6	datasets/routing/routeviews6-prefix2as/YYYY/MM/	routeviews-rv6-YYYYMMDD-HHMM.pfx2as.gz

Each dataset directory also has an index, pfx2as-creation.log, listing the files published in roughly the last thirty days (seqnum, Unix timestamp, path). For older days originet reads the monthly directory listings (YYYY/MM/), which exist back to May 2005 for IPv4 and 2007 for IPv6 (the oldest files are named routeviews-oix-...).

Snapshot selection

• No --date: the newest snapshot in the index.
• --date YYYY-MM-DD inside the index window: the newest snapshot taken on or before that day. If the day itself has no snapshot the closest earlier one is used and a warning names it.
• --date older than the index: the listing of that month, then the previous months, up to twelve, until one has a snapshot on or before the day. A date before 2005 is an error that says so; a gap longer than twelve months is an error that says how far back it looked.

The index is cached and reused for six hours, then fetched again (--refresh forces it). A month's listing is final once it was fetched after that month ended (UTC) and is then kept forever; one fetched while the month was still running follows the six-hour rule. Snapshot files never change once published, so they are cached forever.

The pfx2as format

One line per announced prefix, tab-separated:

1.0.0.0 24  13335
1.1.8.0 24  149511_138421
64.0.0.0    8   64496,64497
2001:4:112::    48  112

The third field is one origin ASN, several joined with _ (a multi-origin prefix, MOAS: several ASes announce it) or several joined with , (an AS set). originet keeps the field verbatim in origins and reports the first ASN as asn. Nothing in the dataset ranks the origins of a MOAS prefix, so treat asn as "one of them" and look at origins (or Match.asns() in Python) when it matters.

Longest-prefix match

Every lookup returns the most specific prefix in the snapshot that covers the address, the same rule routers apply. prefix in the output is that prefix. An address that no prefix covers (private space, unrouted space, or space not seen by RouteViews that day) gets prefix and asn set to null.

An IPv4-mapped IPv6 address (::ffff:8.8.8.8) is looked up as IPv4. A /32 or /128 host suffix is accepted; any other prefix length is rejected, because the tool maps addresses, not prefixes.

Cache

Platform	Default directory
Linux	~/.cache/originet/
macOS	~/Library/Caches/originet/
Windows	%LOCALAPPDATA%\originet\cache\
Docker image	/cache

Override it with --cache-dir or ORIGINET_CACHE_DIR. Inside, files mirror CAIDA's layout: routeviews-prefix2as/pfx2as-creation.log, routeviews-prefix2as/2026/10/routeviews-rv2-20261021-0800.pfx2as.gz, a parsed listing per month visited (routeviews-prefix2as/2015/01/.listing), and the same under routeviews6-prefix2as/. A snapshot is about 4 MB (IPv4) and 1 MB (IPv6) compressed. Downloads go to a temporary file and are renamed into place, so an interrupted run never leaves a truncated snapshot behind.

Nothing expires on its own: every day you ask for stays, about 5 MB per day for both families, and the same day is never downloaded twice. --cache list and --cache size show what is there; --cache prune --keep-days N or --keep-last N (with --dry-run to look first) drops the old snapshots and the month and year directories they leave empty, and never touches the indexes or anything else (see The cache). Deleting by hand works too: a month directory (routeviews-prefix2as/2019/06/) drops its snapshots, the whole cache directory starts over. --fetch --offline --date DAY tells whether a day is cached without downloading.

Terms of use

The dataset is CAIDA's; its terms of use apply to anything you derive from it, and CAIDA asks to be cited (see Citing).