Planning Your Subdomains

The following points are to be considered before dividing your network into a parent and subdomains.

• Number of subdomains. More subdomains means more setup work and more ongoing coordination work for the parent domain administrators in the parent domain. On the other hand, fewer domains mean larger domains. The larger a domain is, the more server speed and memory is required for support.

• Network divisions. You can divide your network into multiple domains however you see fit. Remember that administration is easier if your domain structure follows a consistent, logical, and self-evident pattern.

• Future considerations. The most confusing domain structures have subdomains that are added haphazardly to accommodate new sites and new departments. Try to take future growth into account when designing your domain hierarchy. Take into account stability. Base your subdomains on what is most stable. For example, if your geographic sites are relatively stable but your departments frequently reorganized, base your subdomains on geography rather than organizational function. On the other hand, if your structure is stable but you frequently add sites, base your subdomains on your organizational hierarchy.

• Wide area network (WAN) links. Performance will be better and reliability greater if your domains do not span the WAN. In most cases, WAN links are slower than contiguous network connections and more prone to failure. When servers support machines that can only be reached over a WAN link, more network traffic is funnelled through the slower link. If there is a power failure or other problem at one site, machines at the other sites could be affected. The same performance and reliability considerations apply to DNS zones. As a general rule of thumb, zones should not span WAN links.

• NIS+ compatibility. If your enterprise-level naming service is NIS+, administration is easier if your DNS and NIS+ domain and subdomain structures match.

• Subdomain names. Establish and follow a consistent policy for naming your subdomains. When domain names are consistent, users are more apt to remember and correctly specify the names. Domain names are an important element in all of your DNS data files. If a subdomain name is changed, every file in which the old name appears must be edited. Therefore, choose subdomain names that are stable and unlikely to need changing. Use either full words, such as manufacturing, or abbreviations, such as manf, as subdomain names. However, users might be confused if some subdomains are named with abbreviations and others with full names. If you decide to use abbreviations, use enough letters to clearly identify the name because short cryptic names are hard to use and remember. Do not use reserved top-level Internet domain names as subdomain names. org, net, com, gov, edu, and any of the two-letter country codes such as jp, uk, ca, and it should never be used as a subdomain name.