2.3IPv6 addresses and prefix formats

Born to prepare for IPv4 address exhaustion, IPv6 has a vastly larger 128-bit space and is written in hexadecimal with :: (compressing consecutive zeros), as in 2001:db8::1. When you see IPv6 on the support floor, the first thing you need is to tell the address type apart from its leading bits. This section covers the types—the globally unique global unicast (GUA), the link-only link-local, the internal unique local (ULA), and the self loopback (::1)—the common /64 prefix, and the important difference that IPv6 has no broadcast and substitutes multicast. The goal is identifying types, not fine address calculation.

2.3.1The main types of IPv6 address

• A global unicast (GUA) is in 2000::/3 (the range starting 2000:: through 3fff::) and is routed uniquely on the internet—the equivalent of a public IPv4 address. This is the production address for communicating with the world.
• A link-local (fe80::/10) starts with fe80:: and is an auto-generated address valid only within the same link (same segment). It does not cross a router and cannot reach the internet. Every IPv6-enabled interface always has at least one (the basis for neighbor discovery and the like).
• A unique local (ULA, fc00::/7) is the private equivalent used inside an organization (in practice fd00::/8 is common). It is not routed on the internet and corresponds to private ranges like IPv4's 10.0.0.0/8. A loopback (::1) refers to the device itself (the equivalent of IPv4's 127.0.0.1).

2.3.2Prefixes and the absence of broadcast

• A prefix is the IPv6 version of "network-portion length," the same idea as IPv4's slash notation. Subnets are generally cut at /64 as standard, with the first 64 bits as the network (prefix) and the last 64 bits as the interface identifier. It is written like 2001:db8:0:1::/64.
• IPv6 has no broadcast. The role that broadcast played in IPv4 for "everyone" is handled by multicast (ff00::/8) (starting with ff). A representative example is the all-nodes multicast ff02::1 (to all nodes on the same link), which reaches only the needed group and wastes less.

Suppose you are checking an IPv6-enabled PC and see that its interface has two addresses: fe80::1c2b:5aff:fe33:7a01 and 2001:db8:acad:10::25. In IPv6, one interface commonly holds several addresses at once, and this is not abnormal. Telling the types apart lets you read the situation correctly. The one starting fe80:: is a link-local address, valid only within the same link and auto-generated; so if "the neighbor is reachable but the internet is not," you can hypothesize that only link-local is working while the GUA is not properly configured or advertised. The address starting 2001: is a GUA (in the 2000::/3 range), the production address toward the internet. So for an "cannot reach outside" symptom, the sound move is to check the GUA-side prefix (usually /64), the default route, and the router advertisement (RA) carrying the prefix. Further, carrying over an IPv4 habit—"I want to notify everyone at once, so let me send a broadcast"—is off target in IPv6. IPv6 has no broadcast, and to reach all nodes on the same link you use the all-nodes address ff02::1 of multicast. The key point: just by looking at the leading bits of an address (fe80 / 2000-3fff / fc00-fd / ::1 / ff) you can identify the type and role, and immediately judge "how far this address reaches" and "is this multicast rather than broadcast?" IPv6 looks daunting at first, but for the CCST, mastering this identification of types is enough to hold your own.

2.3.3Section summary

• Tell the type by the leading bits: GUA (2000::/3, internet) / link-local (fe80::/10, same link only) / ULA (fc00::/7, internal) / loopback (::1)
• A prefix is generally /64 (the first half is the network, the second half the interface identifier)
• IPv6 has no broadcast, and the "everyone" role is handled by multicast (ff00::/8, all-nodes ff02::1)