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Chapter 5 of 29

IPv6 Fundamentals: Address Types, Notation, and Basic Configuration

Move beyond IPv4 into IPv6, decoding its notation, address types, and basic configuration so dual-stack environments make sense instead of feeling intimidating.

27 min readen

Why IPv6 Exists and How It Fits Your CCNA World

IPv6 in 2026

IPv6 is widely deployed today in ISPs, mobile, and cloud networks. For CCNA and real jobs, you must be comfortable in dual-stack networks where IPv4 and IPv6 run side by side.

From IPv4 to IPv6

You already know IPv4 addressing, subnetting, and how a default gateway lets hosts reach remote networks. IPv6 keeps these ideas but changes notation, address types, and configuration methods.

Module Goals

You will learn IPv6 structure and shorthand, key unicast types (global, link-local, unique local), SLAAC behavior, basic Cisco IOS configuration, and IPv4 vs IPv6 differences relevant to CCNA.

Big Picture

Think of this as a bridge: IPv6 fixes IPv4 limits (like address exhaustion), removes some mechanisms (like heavy NAT), and adds new ones (like SLAAC and link-local routing).

IPv6 Address Structure and Notation

Basic IPv6 Format

An IPv6 address is 128 bits, written in hex, split into 8 hextets of 16 bits. Example: `2001:0db8:0000:0000:0000:ff00:0042:8329`.

Rule 1: Leading Zeros

You can remove leading zeros in any hextet: `0db8` → `db8`, `0042` → `42`, `0000` → `0`. This makes addresses shorter but still unambiguous.

Rule 2: :: Compression

A single run of all-zero hextets can be replaced by `::`, but only once per address: `2001:db8:0:0:0:ff00:42:8329` → `2001:db8::ff00:42:8329`.

Exam Traps

Watch for `::` used twice (invalid), non-leading zeros removed (wrong), or addresses that cannot expand back to 8 hextets. Always ensure you can restore 8 groups.

Practice: Shorten and Expand IPv6 Addresses

Work through these mentally or on paper. The goal is to build speed and accuracy with IPv6 notation.

  1. Shorten this address using correct rules:
  • `2001:0db8:0000:0000:0000:0000:0000:0001`
  • Step-by-step:
  • Remove leading zeros in each hextet.
  • Then apply `::` once if helpful.
  • Your final answer should be as short as possible but still valid.
  1. Expand this compressed address to its full 8 hextets (4 hex digits each):
  • `fe80::123:4ff:fe56:789a`
  • Steps:
  • Count visible hextets.
  • Insert enough `0000` hextets where `::` appears so the total is 8.
  • Add leading zeros to each hextet so it has 4 hex digits.
  1. Spot the invalid address and explain why:
  • A. `2001:db8::1:0:0:1`
  • B. `fe80::abcd::1`
  • C. `2001:0db8:0:0:8d3:0:0:0`
  1. Challenge: Given `2001:db8:0:0:1234:0:0:1`, write two different incorrect compressions and briefly say what rule they break.

Pause, actually write your answers, then compare to the explanation in your head. Doing this a few times will make IPv6 notation feel natural in dual-stack labs and exam questions.

IPv6 Address Types: Global Unicast, Link-Local, Unique Local

Global Unicast (GUA)

Global unicast addresses are Internet-routable and typically start with `2000::/3` (so they begin with 2 or 3 in hex). Example: `2001:db8:1234:5678::10`.

Link-Local

Link-local addresses start with `fe80::/10` and are only valid on a single Layer 2 link. They are auto-generated and used for neighbor discovery and routing next-hops.

Unique Local (ULA)

Unique local addresses are private IPv6, starting with `fc00::/7` (commonly `fd..`). They are not meant to be globally routed. Example: `fd12:3456:789a::1`.

Pattern Recognition

For CCNA, quickly map prefixes to types: `fe80::` → link-local, `fd..`/`fc..` → unique local, `2xxx`/`3xxx` → global unicast.

Quick Check: Identify the IPv6 Address Type

Classify the following IPv6 address based on its prefix.

You see the address `fe80::1a2b:3c4d:5e6f:7a8b` on a router interface. What type of IPv6 address is this?

  1. Global unicast
  2. Link-local
  3. Unique local
  4. Multicast
Show Answer

Answer: B) Link-local

`fe80::/10` is the reserved prefix for IPv6 link-local addresses. They are only valid on the local link and are used for neighbor discovery and as routing next-hops. Global unicast typically starts with 2 or 3, unique local with fc00::/7 (often fd..), and multicast with ff00::/8.

Stateless Address Autoconfiguration (SLAAC)

What Is SLAAC?

Stateless Address Autoconfiguration (SLAAC) lets IPv6 hosts build their own addresses using information from router advertisements, without needing a DHCPv6 server.

SLAAC Steps

Host creates a link-local address, listens for ICMPv6 Router Advertisements, learns a prefix and default gateway, then combines the prefix with an interface ID to form a global address.

RA Flags

Router Advertisements include flags: A (autonomous) for SLAAC, M (managed) for DHCPv6 addresses, and O (other) for extra info like DNS from DHCPv6.

In Labs

In basic CCNA labs, enabling IPv6 on the router interface is enough for it to send RAs. Hosts then auto-configure global addresses and a default gateway using SLAAC.

Cisco IOS: Basic IPv6 Interface Configuration

Lab Topology

Picture R1–R2 back to back on G0/0, and R1 to a LAN with PCs on G0/1. We will assign IPv6 addresses and enable routing between them.

Enable IPv6 Routing

On each router, enter global config and run: `ipv6 unicast-routing`. This turns on IPv6 forwarding and router advertisements on IPv6-enabled interfaces.

Configure R1 Interfaces

On R1: `int g0/0`, `ipv6 address 2001:db8:0:0::1/64`, `ipv6 address fe80::1 link-local`; then on `g0/1` use `2001:db8:0:1::1/64` and a distinct link-local if desired.

Verify with IOS Commands

Use `show ipv6 interface brief` to see addresses, `show ipv6 route` for the IPv6 routing table, and `ping ipv6 2001:db8:0:0::2` from R1 to test reachability to R2.

Hands-On: Sample IPv6 Configuration and Verification

Study and, if possible, type these commands in a lab environment. This will solidify syntax and show typical outputs you might see on the exam.

IPv4 vs IPv6: Core Differences for CCNA

Size and Format

IPv4 uses 32-bit dotted-decimal addresses; IPv6 uses 128-bit hexadecimal with colons. This massive increase removes address exhaustion concerns.

NAT and Addressing

IPv4 relies heavily on NAT due to limited space. IPv6’s huge space aims for end-to-end addressing, greatly reducing the need for traditional NAT overload.

Config and Discovery

IPv4 typically uses DHCP and ARP with broadcasts. IPv6 uses SLAAC and/or DHCPv6, and replaces ARP with Neighbor Discovery using multicast.

Exam Angle

Expect questions contrasting IPv4 and IPv6 behavior, especially around NAT use, broadcasts vs multicast, and how hosts learn prefixes and gateways.

Quiz: IPv6 Concepts and Cisco Commands

Test your understanding of IPv6 behavior and configuration syntax.

Which of the following statements is MOST accurate for a CCNA-level understanding of IPv6 in a dual-stack network?

  1. IPv6 eliminates the need for any form of NAT, so routers never translate addresses.
  2. Hosts in IPv6 always require a DHCPv6 server to obtain a global unicast address.
  3. Routers use link-local addresses as next-hops, and hosts can use SLAAC to form global unicast addresses from router advertisements.
  4. IPv6 still relies on broadcast for Neighbor Discovery, just like ARP in IPv4.
Show Answer

Answer: C) Routers use link-local addresses as next-hops, and hosts can use SLAAC to form global unicast addresses from router advertisements.

In typical IPv6 deployments, routers and routing protocols use link-local addresses as next-hops, and hosts can use SLAAC to build global unicast addresses from prefixes in router advertisements. IPv6 can still use some forms of NAT (e.g., NAT64), so saying it 'eliminates' NAT is inaccurate. Hosts do not always need DHCPv6 because SLAAC can provide addresses. IPv6 does not use broadcast for Neighbor Discovery; it uses multicast.

Key IPv6 Terms and Patterns

Use these flashcards to reinforce the most testable IPv6 fundamentals.

Standard length of an IPv6 address (in bits)
128 bits, written as eight 16-bit hextets in hexadecimal separated by colons.
Two rules for shortening IPv6 addresses
1) Remove leading zeros in any hextet. 2) Replace a single contiguous run of all-zero hextets with `::` once per address.
Prefix pattern for IPv6 link-local addresses
`fe80::/10` (commonly written as `fe80::` followed by an interface identifier).
Prefix pattern for IPv6 unique local addresses (ULA)
`fc00::/7`, often seen as addresses starting with `fd` for locally assigned ULAs.
Typical prefix range for global unicast IPv6 addresses
`2000::/3`, which practically means addresses starting with 2 or 3 in hex.
Purpose of SLAAC
Allows IPv6 hosts to automatically configure their own global unicast addresses and default gateway using information from router advertisements, without requiring DHCPv6 for addressing.
Cisco global command to enable IPv6 routing
`ipv6 unicast-routing`
Cisco command to view IPv6 addresses on interfaces
`show ipv6 interface brief`
Why IPv6 does not use broadcast
IPv6 replaces broadcast with multicast and uses Neighbor Discovery instead of ARP to reduce unnecessary traffic and improve efficiency.
Role of link-local addresses in routing
Routers use link-local addresses as next-hop addresses for IPv6 routes and for routing protocol neighbor relationships on a local link.

Key Terms

IPv6
The current version of the Internet Protocol with 128-bit addresses, designed to replace IPv4 and provide a vastly larger address space along with simplified header processing.
SLAAC
Stateless Address Autoconfiguration, a mechanism that allows IPv6 hosts to automatically configure their own addresses and default gateway based on router advertisements.
Hextet
A 16-bit block in an IPv6 address, written as four hexadecimal digits and separated from other hextets by colons.
Dual-Stack
A network configuration in which devices run both IPv4 and IPv6 protocols simultaneously, allowing communication over either protocol.
Prefix Length
The number of leading bits in an IPv4 or IPv6 address that represent the network portion, written after a slash (for example, /64 in IPv6).
Link-Local Address
An IPv6 address that is only valid and reachable on a single Layer 2 link, using the fe80::/10 prefix, and commonly used for neighbor discovery and routing next-hops.
Neighbor Discovery
An IPv6 protocol suite that replaces ARP, using ICMPv6 messages and multicast to discover neighbors, resolve link-layer addresses, and manage reachability.
Router Advertisement (RA)
An ICMPv6 message sent by routers to advertise IPv6 prefixes, default gateway information, and configuration flags to hosts.
Unique Local Address (ULA)
An IPv6 address intended for local communications within a site or organization, not routable on the public Internet, using the fc00::/7 prefix.
Global Unicast Address (GUA)
An IPv6 address type that is globally unique and routable on the public Internet, typically starting with 2000::/3.

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