Joe Touch wrote:
Seems clear to me:
RFC1035:
The labels must follow the rules for ARPANET host names. They must
start with a letter, end with a letter or digit, and have as interior
characters only letters, digits, and hyphen. There are also some
restrictions on the length. Labels must be 63 characters or less.
That is syntax in section "2.3.1. Preferred name syntax" as syntax
preferred by applications that use host names.
The following syntax will result in fewer problems with many
applications that use domain names (e.g., mail, TELNET).
The basic rule of the RFC is:
Although labels can contain any 8 bit values in octets that make up a
label
A label may include '.' character as is, as there is no escape
mechanism, though applications expecting host names may fail to
recognize it.
Masataka Ohta