Does Simple DNS Plus support sub-domains (like subname.example.com)?

Yes - there are basically 3 ways to create a sub-domain:

1. If it is a simple sub-domain name that only requires a single host record such as "printer4.example.com" = 192.168.11.22, then you would create it as an A-record in the parent zone (example.com):
   Click the "Records" button, right-click the parent zone in the left pane, select "New host..." from the pop-up menu, and enter the sub-domain name and IP address.

If you have multiple sub-domains which are all hosted on the same web-servers, e-mail-servers etc., you can even use wildcard records ( *.example.com ).
This way you don't have to update the DNS configuration each time you add a new sub-domain.

2. If the sub-domain has multiple records itself (mail, web, ftp, etc.), then you would create it as a separate zone:
   Click the "Records" button, select "New -> Zone", select "Primary zone", click the "Next" button, and the sub-domain name as the zone name, click the "Finish" button.

3. If you want to delegate the sub-domain to another DNS server set (for example if you have a really cool domain name and can sell sub-domains of it), then in the parent zone add NS-records for the sub-domain name pointing to the DNS servers handling it.
   And make sure to add matching A-records (glue records) if the NS-records point to DNS server names which are themselves within the sub-domain.