IBM Support

When does VTAM use Extended Element Addresses?

Question & Answer


SNA allows only one subarea to have 64K (65,536) element addresses. For subarea connections, all VTAM element addresses seen by other subareas (for example, in FID4 PIUs) must be a base (within VTAM's subarea) element. For elements not seen by other subareas (across APPN/FID2/HPR connections), extended (indexed) element addresses can be used by VTAM. These extended element addresses use subarea values of 000xyyyy - where x is an index value above 0, and yyyy is the VTAM subarea number.


For some non-subarea use element addresses, VTAM uses the ENHADDR VTAM start option to determine what type of element address to assign. For some types of resources, VTAM assigns an extended element regardless of the setting of ENHADDR.


VTAM assigns network addresses for all resources that share its subarea. This excludes NCP managed resources, since their network addresses come from the NCP's subarea element pool.

Addresses are assigned in the following ways:

  • For DLUR-served dependent LUs, VTAM uses extended element addresses regardless of the ENHADDR setting.
  • For EE PUs/LINEs, VTAM uses extended element addresses regardless of the ENHADDR setting. Starting with z/OS V1R6, this is also true for HPR PUs.
  • With ENHADDR=YES, PARSESS=NO APPLs in an APPL major node are assigned extended addresses when the major node is activated.
  • With ENHADDR=YES, PARSESS=YES APPLs in an APPL major node, if VTAM has no subarea connections (is a pure Network Node or pure End Node), two extended addresses will be assigned to each APPL when the major node is started.

Reducing the number of addresses

If there are a large number of APPL definitions with a name pattern, then the use of model APPL definitions eliminates the need to predefine these APPLs. If PARSESS=NO, VTAM assigns one network address when the APPL is activated. If PARSESS=YES, VTAM assigns two network address when the APPL is activated. By using model APPL definitions, the assigned network addresses are reduced to only those for active applications. Examples are TN3270 APPLs, session manager APPLs, and TSO APPLs.

If any non-NCP PUs and LUs, or APPLs are defined for future use, but are not currently needed, then deleting these definitions reduces the network addresses that VTAM assigns when these resources are activated.

Monitoring addresses

The only way to monitor element addresses is with the D NET,STATS command. This display includes the number of host element addresses in use at the time and the highest element address assigned. These are displayed for both base addresses and extended element addresses.

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Document Information

Modified date:
15 June 2018