Hi,
in addition to the "always use FQDNs" mantra, I'd like to add that if DNS isn't under your control or isn't fixable due to other issues on layers above 7, you can always help yourself out with the hosts file (/etc/hosts on Unix, windows\system32\drivers\etc\hosts on Windows). On the client, add IPs and FQDNs for cell manager and any potential other server side host (MA host, IS host, GUI host etc). On these server hosts, add the client FQDN. Works wonders.
BTW, I like to install Unix clients locally and then use import from the GUI to get them registered with the CM. Seems to work most reliably. For that matter, I've also extracted the relevant RPMs (CORE, CC, DA are what mostly suffices), put them somewhere wget(1)able, and pull and install them manually. After that, importing the client and an initial Upgrade brings it up to speed. For upgrading, you need a Unix IS though. Ideally you put it on the first Linux box you happen to add (and which is x86_64). Without a Unix IS, patching is really getting nightmarish unless you really have just a single Linux box to deal with.
HTH,
Andre.