IPv6 Terminology

The following list of common terms for network elements and concepts provides a foundation for subsequent chapters. Figure 1-3 shows an IPv6 network.

Figure 1-3. Elements of an IPv6 network

IPv6 common terms and concepts are defined as follows:

• Node

  Any device that runs an implementation of IPv6. This includes routers and hosts.

• Router

  A node that can forward IPv6 packets not explicitly addressed to itself. On an IPv6 network, a router also typically advertises its presence and host configuration information.

• Host

  A node that cannot forward IPv6 packets not explicitly addressed to itself (a non-router). A host is typically the source and a destination of IPv6 traffic, and silently discards traffic received that is not explicitly addressed to itself.

• Upper-layer protocol

  A protocol above IPv6 that uses IPv6 as its transport. Examples include Internet layer protocols such as ICMPv6 and Transport layer protocols such as TCP and UDP (but not Application layer protocols such as FTP and DNS, which use TCP and UDP as their transport).

• Local area network (LAN) segment

  A portion of an IPv6 link consisting of a single medium that is bounded by bridges or Layer 2 switches.

• Link

  One or more LAN segments that are bounded by routers. Many link-layer technologies are already defined for IPv6, including typical LAN technologies (such as Ethernet, Token Ring, and Fiber Distributed Data Interface [FDDI]) and wide area network (WAN) technologies (such as the Point-to-Point Protocol [PPP], Frame Relay, and Asynchronous Transfer Mode [ATM]). Additionally, IPv6 packets can be sent over logical links representing an IPv4 or IPv6 network, by encapsulating the IPv6 packet within an IPv4 or IPv6 header. For more information about LAN and WAN media support for IPv6, see Appendix A, "Link-Layer Support for IPv6."

• Subnet

  One or more links that use the same 64-bit IPv6 address prefix. Another term for subnet is network segment. A subnet can be divided by an intra-subnet router, which is a router providing forwarding and configuration functions for individual links of a subnet. If intra-subnet routers are not present, a subnet is equivalent to a link.

• Network

  Two or more subnets connected by routers. Another term for network is internetwork.

• Neighbors

  Nodes connected to the same link. Neighbors in IPv6 have special significance because of IPv6 Neighbor Discovery, which has facilities to resolve neighbor link-layer addresses and detect and monitor neighbor reachability.

• Interface

  The representation of a physical or logical attachment of a node to a link. An example of a physical interface is a network adapter. An example of a logical interface is a "tunnel" interface that is used to send IPv6 packets across an IPv4 network by encapsulating the IPv6 packet inside an IPv4 header.

• Address

  An identifier that can be used as the source or destination of IPv6 packets that is assigned at the IPv6 layer to an interface or set of interfaces.

• Packet

  The protocol data unit (PDU) that exists at the IPv6 layer and is composed of an IPv6 header and payload.

• Link MTU

  The maximum transmission unit (MTU)—the number of bytes in the largest IPv6 packet—that can be sent on a link. Because the maximum frame size includes the link-layer medium headers and trailers, the link MTU is not the same as the maximum frame size of the link. The link MTU is the same as the maximum payload size of the link-layer technology. For example, for Ethernet using Ethernet II encapsulation, the maximum Ethernet frame payload size is 1,500 bytes. Therefore, the link MTU is 1,500. For a link with multiple link-layer technologies (for example, a bridged link), the link MTU is the smallest link MTU of all the link-layer technologies present on the link.

• Path MTU

  The maximum-sized IPv6 packet that can be sent without performing host fragmentation between a source and destination over a path in an IPv6 network. The path MTU is the smallest link MTU of all the links in the path.