Add reserved code range for private use by applications

See GH/multiformats#158. Changes are analogous to those proposed in GH/multiformats#159.

README.md

@@ -15,6 +15,7 @@
 - [Multicodec table](#multicodec-table)
   - [Adding new multicodecs to the table](#adding-new-multicodecs-to-the-table)
 - [Implementations](#implementations)
+- [Reserved Code Ranges](#reserved-code-ranges)
 - [FAQ](#faq)
 - [Contribute](#contribute)
 - [License](#license)
@@ -62,6 +63,16 @@ The `validate.py` script can be used to validate the table once it's edited.
 - [Ruby](https://github.com/sleeplessbyte/ruby-multicodec)
 - [Add yours today!](https://github.com/multiformats/multicodec/edit/master/table.csv)
 
+## Reserved Code Ranges
+
+The following code ranges have special meaning and may only have meanings assigned to as specified in their description:
+
+### Private Use Area
+
+*Range*: `0x300000 – 0x3FFFFF`
+
+Codes in this range are reserved for internal use by applications and will never be assigned any meaning as part of the Multicodec specification.
+
 ## FAQ
 
 > Why varints?

validate.py

@@ -72,6 +72,12 @@ class CheckError(Exception):
                     )
                 else:
                     codes[code] = name
+
+                # Reserved Code Range: Private Use Area – Do not permit any codes in this range
+                if code in range(0x300000, 0x400000):
+                    raise CheckError(
+                        f"found code in Private Use Area: {hex(code)} with name '{name}'"
+                    )
             except CheckError as e:
                 success = False
                 print(f"row {line}: {e}", file=sys.stderr)