I created a new subdomain: blog.example.com.Sometimes, the blog is accessible without any problems, but other times it's not, just like now at this moment (14-Nov-2013 12:57 PM Lebanon Local Time).Yesterday at night it was working. I've been having this problem for days.If I ping the subdomain, under Windows:Ping request could not be find host blog.example.com. Please check the name and try again.If I try tracert:Unable to resolve target system name blog.example.com.And with nslookup I don't get any records, with my humble knowledge, it seems as DNS problem. But why would it work sometimes? and not other times?Note that the response of the previous commands are instantaneous (in case you need to check some packets or something).I'm also using the Global Google DNS (8.8.8.8). 
Cannot access the subdomain all the time
domains;dns;subdomain;ping
You have a problem with the nameserver (NS) records for beiruthub.com.$ dig NS beiruthub.com;; ANSWER SECTION:beiruthub.com.      1800    IN  NS  dns1.m6.net.beiruthub.com.      1800    IN  NS  dns2.m6.net.beiruthub.com.      1800    IN  NS  dns3.m6.net.beiruthub.com.      1800    IN  NS  ns01.domaincontrol.com.beiruthub.com.      1800    IN  NS  ns02.domaincontrol.com.You are specifying two companies as handling your DNS: m6.net and domaincontrol.com.  This is usually a mistake.  You usually want to have one company handling your DNS.  Otherwise you have to keep your DNS records in sync between two different companies.  Indeed, in your case, the m6.net server knows about blog but the domaincontrol.com server doesn't know about it.$ dig @dns1.m6.net blog.beiruthub.com;; ANSWER SECTION:blog.beiruthub.com. 86400   IN  A   67.228.172.101$ dig @ns01.domaincontrol.com blog.beiruthub.com;; Query time: 165 msec;; SERVER: 216.69.185.1#53(216.69.185.1);; WHEN: Thu Nov 14 07:06:06 2013;; MSG SIZE  rcvd: 115The reason that it works only some of the time is that the DNS entry is sometimes fetched from the m6.net servers (and it works), and is sometimes fetched from the domaincontrol.com servers (and it doesn't work).You will probably want to remove the domaincontrol.com records from your DNS NS records.  They can be edited through your domain registrar.Removing the extra NS records may take a day or more to resolve the problem.  In the meantime you could also add the A record for blog.beiruthub.com to the domaincontrol.com servers so that it can be resolved no matter which name server is chosen.