DNS A-records can be used to configure zone apex domains (also called bare, naked, or root domains). These records have serious availability implications when used in environments such as on-premise data-centers, cloud infrastructure services, and other platforms.
For maximum scalability and resiliency applications should avoid using DNS A-records and instead use a DNS provider that supports CNAME functionality at the apex, or use sub-domains exclusively.
DNS A-records require that an IP address be hard-coded into your application’s DNS configuration. This prevents your infrastructure provider from assigning your app a new IP address on your behalf when adverse conditions arise and can have a serious impact to your app’s uptime.
A CNAME record does not require hard-coded IP addresses and allows Heroku to manage the set of IPs associated with your domain. However, CNAME records are not available at the zone apex and can’t be used to configure root domains.
Always use subdomains, including www.domain.tld