1.6 IPv6 Addressing and Prefix

Why IPv6

IPv6 was created to solve IPv4 exhaustion. Its 128-bit addresses yield about 3.4 x 10^38 addresses. The CCNA tests format, abbreviation, address types, and EUI-64.

Format and abbreviation

An IPv6 address is 128 bits shown as eight groups of four hex digits separated by colons, for example 2001:0DB8:0000:0000:0000:0000:0000:0001. Two rules shorten it:

1. Drop leading zeros within each group: 0DB8 becomes DB8, 0001 becomes 1.
2. Replace one contiguous run of all-zero groups with :: exactly once per address.

Applying both: 2001:0DB8:0000:0000:0000:0000:0000:0001 becomes 2001:DB8::1. You may use :: only once, because a second :: would make the zero count ambiguous.

Address types

Every IPv6-enabled interface automatically gets a link-local (FE80::) address used by neighbor discovery and routing protocols. Important multicast groups: FF02::1 = all nodes, FF02::2 = all routers.

Anycast

An anycast address is assigned to multiple devices; packets go to the nearest one by routing metric. It is used for service redundancy and load distribution. IPv6 has no broadcast at all, anything broadcast did in IPv4 is done by multicast or anycast in IPv6.

EUI-64 interface ID

EUI-64 derives the 64-bit interface ID from a 48-bit MAC:

1. Split the MAC in half and insert FFFE in the middle.
2. Flip the 7th bit (the universal/local bit) of the first byte.

Example: MAC 00:1A:2B:3C:4D:5E becomes interface ID 021A:2BFF:FE3C:4D5E (the 00 first byte flips to 02). Combined with a /64 prefix you get the full address. The giveaway that EUI-64 was used is FFFE in the middle of the interface ID.

Prefix and SLAAC

Most IPv6 subnets use a /64 prefix, leaving 64 bits for the interface ID. Stateless address autoconfiguration (SLAAC) lets a host build its own GUA from a router-advertised prefix plus an EUI-64 or random interface ID, no DHCP server required.

Step-by-step abbreviation practice

Compress 2001:0DB8:0ACD:0000:0000:0000:00B4:0001 yourself:

1. Drop leading zeros per group: 2001:DB8:ACD:0:0:0:B4:1.
2. Replace the longest run of zero groups with ::: 2001:DB8:ACD::B4:1.

Now reverse it: expanding 2001:DB8::1, the :: must restore enough zero groups to total eight, giving 2001:0DB8:0000:0000:0000:0000:0000:0001. If two separate zero runs exist, compress only the longer one; ties pick the leftmost.

Address acquisition methods

A host can obtain an IPv6 address several ways, and the exam expects you to tell them apart:

SLAAC relies on Router Advertisement (RA) messages from Neighbor Discovery Protocol (NDP), the ICMPv6 mechanism that also replaces IPv4 ARP. NDP uses multicast (solicited-node addresses, FF02::1:FFxx:xxxx) rather than broadcast to find neighbors.

IPv6 over IPv4 coexistence

Most networks run dual stack, every device holds both an IPv4 and an IPv6 address and uses whichever the destination supports. Where dual stack is impossible, tunneling carries IPv6 packets inside IPv4. The CCNA emphasizes dual stack as the recommended migration approach.

Common traps

• :: may appear only once, picking the answer with two double-colons is wrong.
• Link-local always starts FE80, and it is mandatory, not optional.
• IPv6 does not use broadcast, any "broadcast" answer for IPv6 is a distractor.
• FFFE in the middle of the interface ID signals EUI-64, not a coincidence.
• SLAAC needs no DHCP server, do not assume an address always came from DHCPv6.

IPv6 address scopes recap

Every IPv6 interface typically holds several addresses at once, and the exam wants you to recognize each by prefix:

• A mandatory link-local (FE80::/10) used for on-link signaling and as the next-hop in routing.
• One or more global unicast (2000::/3) addresses for Internet reachability.
• Possibly a unique local (FD00::/8) address for private internal use.
• Membership in multicast (FF00::/8) groups such as all-nodes (FF02::1).

When you run show ipv6 interface on a router, expect to see both a link-local and a global address listed together, that is normal, not a misconfiguration.

Quick conversions and the /64 boundary

Because almost every IPv6 LAN uses a /64, the first 64 bits are the network prefix and the last 64 bits are the interface ID. This clean split makes EUI-64 and SLAAC possible, the host simply fills the lower 64 bits. A provider commonly delegates a /48 or /56 to a site, which is then carved into many /64 subnets, one per VLAN. Recognizing that /64 is the standard host-subnet size, while shorter prefixes like /48 are site allocations, prevents the common mistake of trying to subnet below /64 for ordinary LANs.