IPv6 Address Types

IPv6 was designed to replace IPv4, and with it came a complete rethinking of addressing. IPv4 had unicast, multicast, and broadcast — but broadcast was inefficient and often unnecessary. IPv6 eliminates broadcast entirely and introduces new address types: Global Unicast (routable on the public internet), Unique Local (private, analogous to RFC 1918), Link-Local (for neighbor discovery and local-link communication, mandatory for every interface), Multicast (one-to-many, replaces broadcast), and Anycast (one-to-nearest, used for load balancing). The exam tests your ability to recognize these types by their prefix and to understand their scope (global, site-local, link-local).

GUAs are globally routable on the internet, similar to public IPv4 addresses. They are assigned by RIRs (like ARIN, RIPE) and typically have a /48 prefix for sites, /64 for subnets. The structure: global routing prefix (48 bits) + subnet ID (16 bits) + interface ID (64 bits). The first 3 bits are 001 (2000::/3), so GUAs always start with 2 or 3 (e.g., 2001:db8::/32 is documentation, 2001:470::/32 is Hurricane Electric). In the exam, if you see an address starting with 2000::/3 (i.e., 2000:: to 3FFF:FFFF:...), it's a GUA. Configuration: either static (ipv6 address 2001:db8:1:1::1/64) or via SLAAC/DHCPv6.

ULAs are the IPv6 equivalent of IPv4 private addresses (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16). They are not meant to be routed on the global internet. Prefix: FC00::/7 (first 7 bits are 1111110), so addresses start with FD (locally assigned) or FC (central assignment, but rarely used). The format: FD + 40-bit global ID (random) + 16-bit subnet ID + 64-bit interface ID. Example: FD12:3456:789A:1::1/64. ULAs are routable within a site but not across the internet. Exam tip: if the address starts with FD, it's ULA.

LLAs are mandatory for every IPv6-enabled interface. They are used for communication on the same link (subnet) for Neighbor Discovery Protocol (NDP), routing protocols (OSPFv3, EIGRP for IPv6), and as the next-hop address for static routes. Prefix: FE80::/10 (first 10 bits are 1111111010), so addresses start with FE80, FE90, FEA0, or FEB0 — but typically you see FE80::. The interface ID is usually derived from the MAC address using EUI-64 (inverting the U/L bit) or randomized for privacy (RFC 4941). LLAs are not routable — they are only valid on the local link. When pinging, you must specify the outgoing interface: ping FE80::1%GigabitEthernet0/0. In the exam, if you see an address starting with FE80, it's link-local.

IPv6 multicast replaces broadcast. Addresses start with FF00::/8 (first 8 bits are 11111111). The second byte defines flags and scope: FF02::/16 is link-local scope (used for NDP), FF05::/16 is site-local. Common multicast addresses: FF02::1 (all nodes), FF02::2 (all routers), FF02::9 (RIPng), FF02::5 (OSPF), FF02::A (EIGRP). Multicast is used for neighbor solicitation, router solicitation, and routing protocols. Exam tip: if the address starts with FF, it's multicast. The scope is critical: FF02 is link-local, FF05 is site-local, FF0E is global.

Anycast is a unicast address assigned to multiple interfaces (typically on different devices). Packets are routed to the nearest interface (by routing protocol metric). There is no special prefix — anycast addresses are taken from the unicast address space (GUA or ULA). The sender cannot distinguish anycast from unicast; the network does the work. Common use: DNS root servers (2001:7fd::1). Exam tip: anycast is not a separate address type by prefix; it's a property of a unicast address. The exam might ask: 'Which address type allows multiple devices to share the same address?' Answer: anycast.

To see IPv6 addresses on a router:

R1# show ipv6 interface brief
GigabitEthernet0/0   [up/up]
    FE80::1
    2001:DB8:1:1::1
GigabitEthernet0/1   [up/up]
    FE80::2
    2001:DB8:2:1::1

To see detailed info:

R1# show ipv6 interface GigabitEthernet0/0
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:DB8:1:1::1, subnet is 2001:DB8:1:1::/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
  ND reachable time is 30000 milliseconds (default)
  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
  Hosts use stateless autoconfiguration for addresses.

IPv6 address types are intimately tied to Neighbor Discovery Protocol (NDP). NDP uses ICMPv6 messages like Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for address resolution (ARP replacement). Router Solicitation (RS) and Router Advertisement (RA) allow hosts to learn the default gateway and prefix. SLAAC (Stateless Address Autoconfiguration) uses RA to assign a GUA. DHCPv6 can provide additional options. Link-local addresses are essential for NDP to work. Multicast addresses (like solicited-node multicast FF02::1:FFxx:xxxx) are used for efficient neighbor discovery.

In a large enterprise campus network, IPv6 is typically deployed with a mix of GUAs and ULAs. For example, a company might receive a /48 prefix from its ISP (e.g., 2001:db8:acad::/48) and subnet it into /64s for each VLAN. Each access layer switch interface is configured with a GUA for user devices and a link-local address for management. The core routers use OSPFv3, which relies on link-local addresses for neighbor adjacency. A common practice is to assign manual link-local addresses (e.g., FE80::1 on each router's loopback) for easier troubleshooting — this way, the OSPF router ID can be set to the loopback's GUA, but the neighbor relationship uses the predictable LLA.

Another scenario: a data center uses anycast for DNS servers. Multiple DNS servers are configured with the same GUA (e.g., 2001:db8:1::53). The network routes to the nearest server based on IGP metric. This provides redundancy and load distribution without complex load balancers. The exam may test this by asking: 'Which address type allows multiple servers to share the same address?' The answer is anycast, but note the address itself is a GUA.

A common misconfiguration is forgetting to enable ipv6 unicast-routing globally. Without it, the router will not forward IPv6 packets between interfaces, even if addresses are configured. Also, using the wrong prefix length (e.g., /48 on a LAN) can cause routing issues because the subnet is too large. In production, always use /64 for subnets to support SLAAC and NDP. If you need smaller subnets for point-to-point links, use /126 or /127, but be aware that some older implementations expect /64. The exam assumes /64 for LANs.

Performance considerations: IPv6 addresses are longer, so routing tables are larger. But modern hardware handles this fine. Security: link-local addresses are often overlooked in ACLs — remember to permit FE80::/10 for routing protocols. Also, RA guard and DHCPv6 guard are used to prevent rogue router advertisements.