Recognise the type from the first digits
| Type | Prefix | Starts with | What it is for |
|---|---|---|---|
| Global unicast (GUA) | 2000::/3 | 2 or 3 | Public addresses, routed on the internet (like public IPv4) |
| Unique local (ULA) | fc00::/7 | fc or fd | Private addresses inside a company (like 10.0.0.0/8); never on the internet |
| Link-local (LLA) | fe80::/10 | fe80 | Only valid on one link (one LAN or one cable); never routed |
| Multicast | ff00::/8 | ff | One packet to a group of devices. IPv6 has no broadcast at all |
| Loopback | ::1/128 | ::1 | The device itself (127.0.0.1 in IPv4) |
| Unspecified | ::/128 | :: | "No address yet", used as a source while a device starts up |
Link-local addresses are everywhere
Every IPv6 interface must have a link-local address, and builds one by itself as soon as IPv6 is on. Routers use link-local addresses to talk to their neighbours, and PCs use the router's link-local address as their default gateway. That's why engineers often set an easy one by hand, such as fe80::1: the same on every interface is fine, because each is only valid on its own link.
Multicast groups you must know
| Address | Who listens |
|---|---|
| ff02::1 | All IPv6 devices on the link |
| ff02::2 | All IPv6 routers on the link |
| ff02::1:ffXX:XXXX | Solicited-node group: devices whose address ends in those 24 bits (used by Neighbor Discovery instead of ARP) |
| ff02::5 / ff02::6 | OSPFv3 routers (later in the course) |
The router below shows its link-local address, its global address, and the groups it joined. Notice FF02::1:FF00:1: the solicited-node group for addresses ending in 00:0001, which covers both FE80::1 and 2001:DB8:ACAD:10::1.
In NetLab
▶ R1 R1>enable R1#show ipv6 interface g0/0 GigabitEthernet0/0 is up, line protocol is up IPv6 is enabled, link-local address is FE80::1 No Virtual link-local address(es): Global unicast address(es): 2001:DB8:ACAD:10::1, subnet is 2001:DB8:ACAD:10::/64 Joined group address(es): FF02::1 FF02::2 FF02::1:FF00:1 MTU is 1500 bytes ICMP error messages limited to one every 100 milliseconds ICMP redirects are enabled ICMP unreachables are sent ND DAD is enabled, number of DAD attempts: 1 ND reachable time is 30000 milliseconds (using 30000) ND advertised reachable time is 0 (unspecified) ND advertised retransmit interval is 0 (unspecified) ND router advertisements are sent every 200 seconds ND router advertisements live for 1800 seconds ND advertised default router preference is Medium Hosts use stateless autoconfig for addresses.
Anycast
An anycast address is a normal unicast address given to several devices (for example DNS servers in different cities). Routing delivers each packet to the nearest one. Nothing in the address itself marks it as anycast.
Remember: Unicast = one device. Multicast = a group. Anycast = the nearest of several. IPv6 has no broadcast: jobs that used broadcast in IPv4 (ARP, DHCP discover) use multicast instead.