The apex domain can only use A records, i.e. point directly to an IP address. It can have multiple A records, ebay.com does so:
host ebay.com
ebay.com has address 66.135.216.190
ebay.com has address 66.211.162.12
ebay.com has address 66.211.181.123
ebay.com has address 66.211.185.25
ebay.com has address 66.211.160.86
ebay.com has address 66.135.209.52
Without a CNAME (alias) record, eBay need to control the DNS resolution. Most people using a CDN don't, so they must use a subdomain.
Ah, I was unaware that there could be multiple IPs on an A record, thanks for that. If I'm understanding this right though, the extra IPs would just be for redundancy and resilience and cannot be relied for geographic routing? In this case ebay.com redirects to www.ebay.com.
It's not that there are multiple IP's in the A record, it's that there are multiple A records, each with an IP address.
For geographic routing, there is a clever trick that can be utilized using a technology called Anycast. Anycast is basically a way of assigning the same IP address to multiple machines so requests to that IP address results in connecting with the one that's the closest to you, route wise.
Providers sometimes use Anycast DNS Name Servers and configure them to provide the different IP addresses depending on which name server people connect to.
So, if someone wants to determine the IP address of ebay, their DNS client connects to ns1.ebay.com and asks "hey, what's the IP addresses for the A records for ebay.com" and ns1.ebay.com replies with the list.
But ns1.ebay.com might be an Anycast DNS Name Server that's close to them and it provides the list of IP addresses closest to that name server. Someone on another continent might reach a name server with the same name and ip address, but it's a different machine in a different data center. It would provide a list of IP addresses on that continent.
I do something similar with one of my sites. I rent three VPS's from buyvm.net (who has Anycast setup) that have the same IP address and are located in Las Vegas, New Jersey, and Luxembourg. I pay less than $10 a month in total and run my DNS name servers there.
Clients that connect to the name server in Las Vegas get an IP pointing to a Digital Ocean load balancer in San Francisco proxying data from a few front-end VPS's.
Clients that connect to the name server in New Jersey get an IP pointing to an OVH Canada load balancer near Montreal.
Clients that connect to the name server in Luxembourg get an IP pointing to an OVH load balancer in the North of France.
The result is a responsive service that has amazingly low latency for the US and the EU. Gonna try to set up some infrastructure in Singapore soon to make things faster for Australia and Asia.
