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+# Base
+
+## Entity ID
+* Room alias: `#<roomAlias>:<serverpart>`
+* Username: `@<username>@<serverpart>`
+* User ID with MSISDN (Mobile Station International Subscriber Directory Number): `%<msisdn without +>:<serverpart>`
+* User ID with Email: `^<email username>_at_<email hostname>:<serverpart>`
+* Message ID: `&<uuid>:<serverpart (from which server the message was sent)>`
+* Room ID: `!<roomID>@<serverpart>`
+* Single server-part: `<serverpart>`
+
+**Server-part (hostname)** - `IPv4 / [IPv6] / dns-domain:<port (1-65535)>`.  
+**Username/Room alias/RoomID** - MUST NOT be empty, and MUST contain only the characters `a-z`, `0-9`, `.`, `_`, `=`, `-`, and `/`.
+
+RoomID SHOULD be UUID identifier.
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