Once you've decided how many subdomains you'd like to create and what they correspond to, you should choose good names for them. Rather than unilaterally deciding on your subdomains' names, it's considered polite to involve your future subdomain administrators and their constituencies in the decision. In fact, you can leave the decision entirely to them, if you like.
This can lead to problems, though. It's nice to use a relatively consistent naming scheme across your subdomains. It makes it easier for users in one subdomain, or outside your domain entirely, to guess or remember your subdomain names, and to figure out in which domain a particular host or user lives.
Leaving the decision to the locals can result in naming chaos. Some will want to use geographical names, others will insist on organizational names. Some will want to abbreviate, others will want to use full names.
Therefore, it's often best to establish a naming convention before choosing subdomain names. Here are some suggestions from our experience:
In a dynamic company, the names of organizations can change frequently. Naming subdomains organizationally in a climate like this can be disastrous. One month the Relatively Advanced Technology (RAT) group seems stable enough, the next month they've been merged into the Questionable Computer Systems organization, and the following quarter they're all sold to a German conglomerate. Meanwhile, you're stuck with well-known hosts in a subdomain whose name no longer has any meaning.
Geographical names are more stable than organizational names, but sometimes not as well known. You may know that your famous Software Evangelism Business Unit is in Poughkeepsie or Waukegan, but people outside your company may have no idea where it is (and might have trouble spelling either name).
Don't sacrifice readability for convenience. Two-letter subdomain names may be easy to type, but impossible to recognize. Why abbreviate "Italy" to "it" and have it confused with your Information Technology organization, when for a paltry three more letters you can use the full name and eliminate any ambiguity?
Too many companies use cryptic, inconvenient domain names. The general rule seems to be: the larger the company, the more indecipherable the domain names. Buck the trend: make the names of your subdomains obvious!
Don't use existing or reserved top-level domain names as subdomain names. It might seem sensible to use two-letter country abbreviations for your international subdomains, or to use organizational top-level domain names like net for your networking organization, but it can cause nasty problems. For example, naming your Communications department's subdomain com might impede your ability to communicate with hosts under the top-level com domain. Imagine the administrators of your com subdomain naming their new Sun workstation sun and their new HP 9000 hp (they aren't the most imaginative folks): users anywhere within your domain sending mail to friends at sun.com or hp.com could have their letters end up in your com subdomain, since the name of your parent zone may be in some of your hosts' search lists.
 Actually, not all mailers have this problem, but some popular versions of sendmail do. It all depends on which form of canonicalization it does, as we discussed in the section entitled "Electronic Mail" in Chapter 6, Configuring Hosts.