It depends, if you are at shared hosting and using the hosting company name servers you may not have any access to DNS management.
For me, my domain names were purchased through Yahoo domains and they provide a web page for managing DNS records. I can make up arbitrarynames.mojoportal.com and point them at whatever ip address I want. Network solutions and other domain name vendors also provide tools for DNS management, so it depends who your registrar is for your domain and whether they provide a way to manage DNS.
Often shared hosting providers will ask you to transfer your domain to them and then it uses their name servers and often you have no control at that point and have to get your host to make DNS records. Often in shared hosting even if you have a fixed ip address it is not a dedicated ip address, that means other sites use the same ip address as your site and in this case the host also sets up IIS host headers to resolve the host name to your specific site. So in that scenario you would also need host headers in IIS for each subdomain.
A dedicated ip address means you can reach the site just by using the ip address like http://123.12.123/ or whatever the ip address is. A dedicated ip address is what you need if you don't have control over IIS and can't add IIS host headers. With a dedicated ip address it is sufficient to point the DNS a record for each sub domain at the ip address. If it is a shared fixed ip address then you also need to be able to add IIS host headers to tell IIS which site handles that host name.
Hope it helps,