There are can two configurations based on actual need.
Wildcard SSL Certificates:
Wildcard SSLs are of many types, some of them support even different IP address/servers as long as your domain name remains same upto unlimited number of sub-domains.
How URL looks: https://sub-domain.yourdomainname.extension
When to use?
- To product multiple sub-domains with single SSL certificate
- When Multiple sub-domains are part of single domain name (Generally used by companies)
- Examples, erp.yourdomain.ext, secure.yourdomain.ext, shop.yourdomain.ext, cp.yourdomain.ext, www.yourdomain.ext, yourdomain.ext etc.
- Certificate can be Domain Validation (Green Lock) or Business Validation (Green Bar)
- Costs approx. between $$ to $$$
If you are willing to secure, only few sub domains of your same domain name hosted across various servers, then probably SSL Certificate for single domain name/without wildcard can be best option, as you can buy couple of different certificates for each sub-domain from different/single vendors at very cheaper rates compared to wildcard.
Such SSL certificate can secure subdomain.domainame.ext, ex. www.domainname.ext and/or domainname.ext