How VTAM handles network and subarea addressing

For VTAM® to control routing in a subarea network or within a composite network node, VTAM must know the location of its resources, such as logical units, physical units, and other SSCPs. VTAM uses element addresses, in conjunction with the subarea address, to identify the location of resources (also known as network addressable units). The subarea address indicates in which subarea the resource is located; the element address indicates the unique address within the subarea.

Minor nodes, such as an application program or logical unit, require at least one element address. Some minor nodes, such as local non-SNA devices and application programs that use parallel sessions, require more than one element address. This can increase the number of element addresses that is used in a subarea.

The different types of addressing structures are:

Preextended network addressing: Used by releases before VTAM Version 3 Release 1. Enables you to define 255 subareas and 254 elements.
Extended network addressing (ENA): Used by VTAM Version 3 Release 1 and subsequent releases before VTAM Version 3 Release 2. ENA supports addressing to 255 subareas and extends element addressing to 32 768 elements. Beginning with VTAM Version 4 Release 1, host element addressing was extended to 65 535 elements.
Extended subarea addressing: Used by VTAM Version 3 Release 2 (with compatibility PTF) and subsequent releases. Extended subarea addressing increased the size of the subarea address to 65 535 and the number of explicit routes for each destination subarea to 16.
Note: Applications are assigned a low-order primary LU address upon activation. This low-order address may be used in cases where a high-order address could have been used.