Every SNA Server computer is a member of a Windows NT domain. The primary domain controller in the Windows NT domain manages user accounts and controls access to network resources.
Every SNA Server computer is a member of an SNA Server subdomain. An SNA Server subdomain is a collection of SNA Server computers that share common configuration information and, to the SNA Server client, appear to function as a unit, providing a single set of resources (such as LUs and LU pools). Unlike Windows NT domains, SNA Server subdomains form a logical grouping of SNA Server computers through which SNA Server clients may connect to an SNA host. SNA Server subdomains play no role in Windows NT user authentication.
Because they rely on Windows NT domain security to control access to the network, however, all computers in a particular SNA Server subdomain must belong to the same Windows NT domain. In addition, trust relationships must be established between the appropriate Windows NT domains if users will be logging on to a domain other than the one containing the SNA Server computers to which the users will be connecting.
One consideration in determining the scope of a subdomain and the location of its members is the capacity of the network link that connects the members. Whenever the subdomain configuration is changed (by adding a new user or LU, for example), the changed configuration file is propagated to all SNA Server computers whose role is set to "backup."
Most often, a subdomain should be contained within a local LAN segment or ring to avoid unnecessary traffic across slower WAN connections. Using the deployment models as guidelines, SNA Server subdomains typically encompass SNA Server computers found at the branch offices or centrally near the host.
Another consideration in determining the scope of a subdomain is the restriction that it may contain no more than 15 servers. Note, however, that the number of subdomains that can be contained within a Windows NT domain is virtually unlimited.