This ?¬?eld allows an
organization to use up to 65,535 individual subnets. For more information, refer to RFC
3587, ???IPv6 Global Unicast Address Format,??? which replaces RFC 2374.
IPv6 over Data Link Layers
IPv6 is de?¬?ned on most of the current data link layer protocols, including the following
protocols:
?– Ethernet*
?– PPP*
?– High-Level Data Link Control (HDLC)*
?– FDDI
?– Token Ring
?– Attached Resource Computer network (ARCnet)
?– Nonbroadcast multiaccess (NBMA)
?– ATM**
?– Frame Relay***
?– IEEE 1394
* Cisco supports these data link layers.
** Cisco supports only ATM permanent virtual circuit (PVC), not switched virtual
circuit (SVC) or ATM LAN Emulation (LANE).
***Cisco supports only Frame Relay PVC, not SVC.
An RFC describes the behavior of IPv6 in each of these speci?¬?c data link layers, but Cisco
IOS Software does not necessarily support all of them. The data link layer de?¬?nes how IPv6
interface identi?¬?ers are created and how neighbor discovery deals with data link layer
address resolution.
Larger address spaces make room for large address allocations to ISPs and organizations.
An ISP aggregates all the pre?¬?xes of its customers into a single pre?¬?x and announces the
single pre?¬?x to the IPv6 Internet.
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