mod_rewrite 
; that is so Mickey Mouse and slows the server ...
It can be easily done in the DNS template for the site (the correct way);
Code:
$TTL 14400
domain-name.com. 86400 IN SOA ns1.domain-name.com. emailname.hotmail.com. (
2011110109 ;Serial Number
86400 ;refresh
7200 ;retry
3600000 ;expire
86400 ;minimum
)
domain-name.com. 86400 IN NS ns1.domain-name.com.
domain-name.com. 86400 IN NS ns2.domain-name.com.
domain-name.com. 14400 IN A 100.100.100.100
localhost 14400 IN A 127.0.0.1
domain-name.com. 14400 IN MX 0 domain-name.com.
mail 14400 IN CNAME domain-name.com.
www 14400 IN CNAME domain-name.com.
ftp 14400 IN A 100.100.100.100
ns1 14400 IN A 100.100.100.100
ns2 14400 IN A 200.200.100.100
m.domain 14400 IN A 100.100.100.100
www.m.domain 14400 IN A 100.100.100.100
{....}_EOF_
Either sub-domain.com or www.sub-domain.com will work this way -- www is a "sub-domain" in itself so sub-domain.sub-sub-domain.domain.tld really makes little sense IMHO.