WWW subdomain not working why?

Question

Apache HTTP Server

Hello.
Tell me what could be the problem?
When you go to a subdomain of a site without www, the site opens.
When you go to the subdomain of the site from www, the site does not respond, when you ping it does not see DNS.
At the same time, in the "A" cloud panel, the entries are on www + subdomain + main site
A in the Apache config, in the ServerAlias ​​sections, subdomains with www and without www are indicated.
I can't find the reason, can anyone help?
All this is necessary in order to redirect from www to without www in the future, in case someone (perhaps) hits the site's subdomain via WWW.

Answer 1

For a domain, use the A record in DNS, for subdomains, use the CNAME record.