## Reference

```typescript
type Hostname = string;
type Id = Entity["id"];

type Reference = `${Hostname}:${Id}` | `${Id}`;
```

A **reference** is a way to refer to any entity within the Versia network. It is a string composed of the following parts:

- The entity's instance's hostname. Optional if that same instance is the creator of the entity.
    - Punycode encoding is used for internationalized domain names (IDNs).
    - Example: `example.com`, `example.com:3000`, `xn--ls8h.xn--ls8h`.
- A colon (`:`) separator, if the hostname is present.
- The entity's unique identifier. This is the `id` property of the entity.

<Warning>
    If the hostname is an IPv6 address, it must be enclosed in square brackets.
    
    For example: `[2001:db8::1]:3000`.
</Warning>


### Examples

These two examples are equivalent if the instance is `example.com`:

```jsonc
{
    "type": "Follow",
    "id": "3e7e4750-afd4-4d99-a256-02f0710a0520",
    "author": "6e0204a2-746c-4972-8602-c4f37fc63bbe", // [!code focus]
    "created_at": "2021-01-01T00:00:00.000Z",
    "followee": "test.org:02e1e3b2-cb1f-4e4a-b82e-98866bee5de7"
}
```

```jsonc
{
    "type": "Follow",
    "id": "3e7e4750-afd4-4d99-a256-02f0710a0520",
    "author": "example.com:6e0204a2-746c-4972-8602-c4f37fc63bbe", // [!code focus]
    "created_at": "2021-01-01T00:00:00.000Z",
    "followee": "test.org:02e1e3b2-cb1f-4e4a-b82e-98866bee5de7"
}
```

## RFC3339

[https://datatracker.ietf.org/doc/html/rfc3339](https://datatracker.ietf.org/doc/html/rfc3339)

## UUID

```typescript
type UUID = `${number}-${number}-${number}-${number}-${number}`;
```

## URI

```typescript
type URI = string;
```

## Extensions

```typescript
type OrgNamespace = string;
type ExtensionName = string;

type ExtensionsKey = `${OrgNamespace}:${ExtensionName}`;

type Extensions = {
    [key in ExtensionsKey]: any;
}
```