Can't set non-/64 mesh-local prefixes with ot-cli

[spacekitteh]

Describe the bug A clear and concise description of what the bug is.
As far as I'm aware, mesh-local prefixes aren't required to be /64s. When trying to set a non-/64 prefix, ot-cli fails to parse it.

E.g.:

> dataset meshlocalprefix fd00:db8:3::/48    
Error 6: Parse
> dataset meshlocalprefix [fd00:db8::]/16
Error 6: Parse

[abtink]

The mesh-local prefix should be 64 bits long, as it is used to construct RLOC, ALOC, and ML-EID addresses. Section 8.10.1.8 (in the Thread specification), where the "Network Mesh-Local Prefix TLV" is defined, explicitly states "length = 8".

The CLI command dataset meshlocalprefix does not require the prefix length (/64) to be specified when setting the mesh-local prefix. Here are links to its documentation, which provide examples:

• README_DATASET.md#meshlocalprefix
• openthread.io CLI documentation

---

This is also reflected in the code and definition of otMeshLocalPrefix:

#define OT_MESH_LOCAL_PREFIX_SIZE OT_IP6_PREFIX_SIZE ///< Size of the Mesh Local Prefix (bytes)

/**
 * Represents a Mesh Local Prefix.
 */
typedef otIp6NetworkPrefix otMeshLocalPrefix;

and

#define OT_IP6_PREFIX_SIZE 8                            ///< Size of an IPv6 prefix (bytes)
#define OT_IP6_PREFIX_BITSIZE (OT_IP6_PREFIX_SIZE * 8)  ///< Size of an IPv6 prefix (bits)

[jwhui]

RFC 4944 Section 6 states:

An IPv6 address prefix used for stateless autoconfiguration RFC4862
of an IEEE 802.15.4 interface MUST have a length of 64 bits.