So I have a domain name registered with GoDaddy. I also have multiple routers with different dynamic IPs that I want to access with this domain. For example, site1.mydomain.com, site2.mydomain.com. I used to have DynDNS service, but it's been a while and I heard they got rid of their free service.

What's the easiest and cheapest way to do this?

 

Add a CNAME in your domain's DNS to point to the DDNS hostname, for example:

dynamic IN CNAME myhost.no-ip.com

http://www.ntchosting.com/dns/cname.html

Plenty of DDNS providers out there now.

 

I've seen several people say to add a CNAME, but it needs to be pointed to an A record. Why can't I just use the A record as my DDNS hostname?

 

You're mapping one domain name to another. An A record is a host within one domain. So if you own mydomain.com and you want router1.mydomain.com to go to router1.no-ip.com, a CNAME is the only DNS record type that will point outside your own domain.

Why does it have to pointed to an A record? The DDNS provider is going to have the A record (which is dynamically updated) and that A record will be in their own domain. You're just adding a secondary name pointing to their host in their domain. If that's a GoDaddy DNS Manager limitation, you may need to contact a GoDaddy tech to make the entry behind the scenes.