A zone includes hosts located in more than one building or site, yet at least one of the authoritative name servers supporting the zone is not as geographically and topologically distributed as the most remote host.

When authoritative name servers are co-located in the same facility, the loss of the facility likely leads to the loss of access to all servers defined in their zones (i.e., nobody can resolve their names). If one or more of the hosts in the supported zones are located at a different site, they would be effectively down even though they would otherwise be fully operational. This scenario can only be prevented with geographic dispersal of name servers. Organizations should also be prepared for greater disasters, such as the destruction of a building, an entire campus, or in the case of a hurricane, an entire city. In situations in which all the hosts defined on an authoritative name server are located in the same building as the name server, then loss of DNS will not impact availability of service simply because the computing infrastructure is already down. On the other hand, if all the authoritative name servers for a zone reside in a single building, but hosts defined within the zone are located elsewhere, then the loss of the DNS will impact service. The loss of service occurs because users (and other infrastructure devices and servers) will not be able to resolve host names for servers/services that are otherwise still operational at an unaffected site.