Yes, but doing this will require a ‘work-around’.
Once your DreamHost account has been approved, you will need to add the sub-domain to the DreamHost hosting system via the panel. However, the panel will not allow you to add the sub-domain to the system without first adding the parent domain, this is the ‘work around’ I mentioned. Note, adding the parent domain will not affect the hosting of the parent domain elsewhere, it is only required so you can add the sub-domain without error.
Once you have the sub-domain in the system, you can obtain the IP address assigned to it via the panel and configure the DNS at your registrar to point at this IP. Another note; each physical server at DreamHost allocates IP addresses from a pool of IP addresses (and Apache instances), so each domain or sub-domain you have hosted is likely to end up on a different IP address. Therefore, it is not possible to just obtain the IP address of the physical server, as I said, you need to obtain the actual IP address that has been allocated to the sub-domain via the panel and I should also note that this IP address is subject to change, although in reality this doesn’t seem to happen too often.
Well, you won’t be able to specify the sub-domain during the sign-up process and if you intend to keep the parent domain hosted elsewhere, then it is not wise to specify that either. You best options would be to either take advantage of the ‘free’ domain registration included with every plan, or specify a ‘free’ dreamhosters.com sub-domain, then add your real sub-domain to the system once everything is setup.
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