NIP-73
======

External Content IDs
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`draft` `optional`

There are certain established global content identifiers such as [Book ISBNs](https://en.wikipedia.org/wiki/ISBN), [Podcast GUIDs](https://podcastnamespace.org/tag/guid), and [Movie ISANs](https://en.wikipedia.org/wiki/International_Standard_Audiovisual_Number) that are useful to reference in nostr events so that clients can query all the events assosiated with these ids.


`i` tags are used for referencing these external content ids, with `k` tags representing the external content id kind so that clients can query all the events for a specific kind.

## Supported IDs

| Type | `i` tag | `k` tag |
|- | - | - |
| URLs | "`<URL, normalized, no fragment>`" | "`<scheme-host, normalized>`" |
| Hashtags | "#`<topic, lowercase>`" | "#" |
| Geohashes| "geo:`<geohash, lowercase>`" | "geo" |
| Books | "isbn:`<id, without hyphens>`" | "isbn" |
| Podcast Feeds | "podcast:guid:`<guid>`" | "podcast:guid" |
| Podcast Episodes | "podcast:item:guid:`<guid>`" | "podcast:item:guid" |
| Podcast Publishers | "podcast:publisher:guid:`<guid>`" | "podcast:publisher:guid" |
| Movies | "isan:`<id, without version part>`" | "isan" |
| Papers | "doi:`<id, lowercase>`" | "doi" |
| Nostr Event ID | "e:`<event-id>`" | "e" |
| Nostr Event Address | "a:`<address>`" | "a" |
| Nostr Pubkey | "p:`<pubkey>`" | "p" |

Note that nostr-native IDs should only be used when avoiding a collision with other tags with the same type, for example with [NIP 85](./85.md) reviews that tag a pubkey in the review.

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## Examples

### Books:

- Book ISBN: `["i", "isbn:9780765382030"]` - https://isbnsearch.org/isbn/9780765382030

Book ISBNs MUST be referenced _**without hyphens**_ as many book search APIs return the ISBNs without hyphens. Removing hypens from ISBNs is trivial, whereas adding the hyphens back in is non-trivial requiring a library.

### Podcasts:

- Podcast RSS Feed GUID: `["i", "podcast:guid:c90e609a-df1e-596a-bd5e-57bcc8aad6cc"]` - https://podcastindex.org/podcast/c90e609a-df1e-596a-bd5e-57bcc8aad6cc
- Podcast RSS Item GUID: `["i", "podcast:item:guid:d98d189b-dc7b-45b1-8720-d4b98690f31f"]`
- Podcast RSS Publisher GUID: `["i", "podcast:publisher:guid:18bcbf10-6701-4ffb-b255-bc057390d738"]`

### Movies:

- Movie ISAN: `["i", "isan:0000-0000-401A-0000-7"]` - https://web.isan.org/public/en/isan/0000-0000-401A-0000-7

Movie ISANs SHOULD be referenced  _**without the version part**_ as the versions / edits of movies are not relevant. More info on ISAN parts here -  https://support.isan.org/hc/en-us/articles/360002783131-Records-relations-and-hierarchies-in-the-ISAN-Registry

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### Optional URL Hints

Each `i` tag MAY have a url hint as the second argument to redirect people to a website if the client isn't opinionated about how to interpret the id:

`["i", "podcast:item:guid:d98d189b-dc7b-45b1-8720-d4b98690f31f", https://fountain.fm/episode/z1y9TMQRuqXl2awyrQxg]`

`["i", "isan:0000-0000-401A-0000-7", https://www.imdb.com/title/tt0120737]`