Interface identifiers in IPv6 unicast addresses are used to identify interfaces on a link. They are required to be unique on that link. In some cases, an interface's identifier is derived directly from that interface's link-layer address. z/OS® Communications Server does not allow two links to have the same local address. Some implementations might allow the same interface identifier to be used on multiple interfaces on a single node, as long as they are attached to different links.

z/OS Communications Server allows the interface identifier to be generated (the default) or manually configured. When the interface ID is generated, then z/OS builds the interface ID when the interface becomes active based on the interface type as follows:

• OSA-Express QDIO
  1. OSA-Express returns the MAC address and a unique instance value during the start of an interface.
  2. z/OS builds the interface identifier by inserting the unique instance value into the middle of the MAC address. This ensures that when multiple stacks share an OSA, each stack gets a unique interface ID. If a virtual MAC address is configured for this interface, then z/OS instead inserts the value 'FFFE'x into the middle of the MAC address.
• HiperSockets™

  For HiperSockets interfaces, the interface ID generation works the same as for OSA-Express QDIO except that the HiperSockets device returns a 48-bit value that is unique for the HiperSockets CHPID rather than a MAC address. This ensures that when multiple stacks share a HiperSockets CHPID, each stack gets a unique interface ID.

• MPCPTP6

  For MPCPTP6 interfaces, z/OS randomly generates an interface ID.

  Figure 1. OSA-Express QDIO interface ID format

  

A node can choose to use a different algorithm available for generation of interface identifiers for IPv6 addresses on a different platform.