Internet Protocol Version 6

Internet Protocol Version 6 (IPv6) is the next generation of the Internet protocol designed to replace the current version, Internet Protocol Version 4 (IPv4). Most of today's internets use IPv4, for which there is a growing shortage of addresses. In theory, 32 bits provide over 4 billion nodes, each with a globally unique address. In practice, the interaction between routing and addressing makes it impossible to use more than a small fraction of that number of nodes. Consequently, there is a growing concern that the continued growth of the Internet might lead to the exhaustion of IPv4 addresses early in the 21st century.

IPv6 fixes a number of problems in IPv4, such as the limited number of available IPv4 addresses. IPv6 uses 128-bit addresses, an address space large enough to last for the foreseeable future. It also adds many improvements to IPv4 in areas such as routing and network autoconfiguration. IPv6 is expected to gradually replace IPv4, with the two coexisting for a number of years during a transition period.

IPv6 is an evolutionary step from IPv4. Functions that work well in IPv4 were kept in IPv6, and functions that did not work well in IPv4 were removed.

z/OS® Communications Server Version 1 Release 4 was the first release to incorporate IPv6 features. With z/OS Communications Server, you can accomplish the following tasks:

Build an IPv6 network
Start using IPv6-enabled applications
Enable existing IPv4 applications to be IPv6 applications
Access your SNA applications over an IPv6 network

Not all IPv6 features are supported by z/OS. This information describes the support available and how to implement it.

IPv6 provides the following advantages.

Expanded routing and addressing

IPv6 uses a 128-bit address space, which has no practical limit on global addressability and provides 3.4 × 10⁵⁰ unique addresses. This provides enough addresses so that every person could have a single IPv6 network with many nodes, and still the address space would be almost unused.

The greater availability of IPv6 addresses eliminates the need for private address spaces, which in turn eliminates one of the needs for network address translators (NATs) to be used between the private intranet and the public Internet.

Hierarchical addressing and routing infrastructure

The use of hierarchical address formats is equally important as the expanded address space. The IPv4 addressing hierarchy includes network, subnet, and host components in an IPv4 address. With its 128-bit addresses, IPv6 provides globally unique and hierarchical addressing based on prefixes rather than address classes, which keeps routing tables small and backbone routing efficient.

The general format is shown in the following figure:

Figure 1. IPv6 address space

First n bits are the global routing prefix; next m bits are the subnet ID; last 128-(n+m) bits are the interface ID.

The global routing prefix is a value (typically hierarchically structured) assigned to a site; the subnet ID is an identifier of a link within the site; and the interface ID is a unique identifier for a network device on a given link (usually automatically assigned).

Simplified IP header format

The IPv6 header has a fixed size and its format is more simplified than the IPv4 header. Some fields in the IPv4 header were dropped in IPv6 or moved to optional IPv6 extension headers to reduce the common-case processing cost of packet handling, as well as keep the bandwidth cost of the IPv6 header as low as possible despite increasing the size of addresses. While the IPv6 address is four times the size of the IPv4 address, the total IPv6 header size is only twice as large as the IPv4 header size.

Improved support for options

Changes in the way IP header options are encoded allows for more efficient forwarding, less stringent limits on the length of options, and greater flexibility for introducing new options in the future. Optional IPv6 header information is conveyed in independent extension headers located after the IPv6 header and before the transport-layer header in each packet. In contrast to IPv4, most IPv6 extension headers are not examined or processed by intermediate nodes.

Address autoconfiguration

IPv6 provides for both stateless and stateful autoconfiguration. Stateless autoconfiguration allows a node to be configured in the absence of any configuration server. Stateless autoconfiguration also makes it possible for a node to configure its own globally routable addresses in cooperation with a local IPv6 router, by combining the 48- or 64-bit MAC address of the adapter with network prefixes that are learned from the neighboring router.

IPv6 allows the use of DHCPv6 for stateful autoconfiguration. DHCPv6 relies on a configuration server that maintains static tables to determine the addresses that are assigned to newly connected nodes. z/OS Communications Server does not support DHCPv6.

Tip: You can manually configure addresses in environments in which complete local control is required (as with VIPA or additional LOOPBACK addresses).

Dual-mode stack support

z/OS Communications Server can be an IPv4-only stack or a dual-mode stack. Dual-mode stack refers to a single TCP/IP stack supporting both IPv4 and IPv6 protocols at the same time.

Restriction: IPv6-only stacks are not supported.

Running in a dual-mode stack configuration provides the following advantages:

IPv4 and IPv6 applications can coexist on a single dual-mode stack.
Unmodified applications can continue to send data over an IPv4 network.
A single IPv6-enabled application can communicate using IPv4 and IPv6.
IPv4 and IPv6 can coexist in the same devices and networks.