I can't set a DNS name that starts with a digit.

I can confirm this behavior.

RFC 1178 recommends not using DNS Host names that start with numbers. https://tools.ietf.org/html/rfc1178 here is the relevant section:


Don't use digits at the beginning of the name.

Many programs accept a numerical internet address as well as a
name. Unfortunately, some programs do not correctly
distinguish between the two and may be fooled, for example, by
a string beginning with a decimal digit.

Names consisting entirely of hexadecimal digits, such as
"beef", are also problematic, since they can be interpreted
entirely as hexadecimal numbers as well as alphabetic strings.

Don't use non-alphanumeric characters in a name.

Your own computer may handle punctuation or control characters
in a name, but most others do not. If you ever expect to
connect your computer to a heterogeneous network, you can count
on a variety of interpretations of non-alphanumeric characters
in names. Network conventions on this are surprisingly
nonstandard.

Yep, funny that this still comes up. I guess this must still be true after all these years.

There was conjecture in the "old days" that www.3com.com was the only exception ever made. I have no idea if that's true but it made a good story. They are long gone now of course.

(Edit: It's not true. I just thought of at least 1 other.)

Seems like Mikrotik is not RFC compliant here.
https://tools.ietf.org/html/rfc1123#page-13 (note that this is an INTERNET STANDARD RFC, not just INFORMATIONAL as was previously linked, which was a copy of a magazine article)