All single octet length top level domain (TLD) names are reserved.
    Should the root zone ever get very large, there are technical
    solutions involving referral to servers providing splits of the zone
    based on the first name octet, which would be eased by having the
    single byte TLDs available.  In addition, these provide a potential
    additional axis for DNS expansion.  For like reasons, it is
    recommended that within TLD zones or indeed within any zone that is
    or might become very large, in the absence of a strong reason to the
    contrary, all single octet names be reserved.