IPv6 Fundamentals Complete Guide

IPv6 address format and notation

An IPv6 address contains 128 bits written as eight hexadecimal groups. Leading zeros in a group can be omitted, and one consecutive sequence of all-zero groups can be compressed with ::. The double-colon rule can be used only once in an address.

Expanded	Compressed
2001:0db8:0000:0000:0000:ff00:0042:8329	2001:db8::ff00:42:8329
fe80:0000:0000:0000:021c:7eff:fe12:3456	fe80::21c:7eff:fe12:3456
0000:0000:0000:0000:0000:0000:0000:0001	::1

Prefix length uses CIDR notation such as /64. A /64 is the standard subnet size for many ordinary LANs because multiple IPv6 mechanisms assume a 64-bit interface identifier.

Important IPv6 address types

Type	Prefix/example	Purpose
Global unicast	Commonly within 2000::/3	Globally routable unicast addressing.
Link-local	FE80::/10	Local-link communication, neighbour discovery and routing-protocol next hops.
Unique local	FC00::/7, commonly locally assigned FD00::/8	Private-like internal addressing, not intended for global Internet routing.
Loopback	::1/128	Local host.
Unspecified	::/128	No address assigned or source not yet known.
Multicast	FF00::/8	One-to-many group delivery.
Anycast	Uses unicast address format	Traffic reaches one suitable instance, often the nearest by routing.

No broadcast: IPv6 uses multicast and other mechanisms instead of an IPv4-style broadcast address.

IPv6 header and extension headers

The fixed IPv6 header is streamlined and includes version, traffic class, flow label, payload length, next header, hop limit, source and destination. Optional functions are carried in extension headers. Intermediate routers do not perform IPv6 fragmentation; the source uses a Fragment extension header when required.

The hop limit replaces IPv4 TTL terminology. The next header identifies either an extension header or upper-layer protocol such as TCP, UDP or ICMPv6.

Neighbour Discovery Protocol

IPv6 NDP uses ICMPv6 for functions that IPv4 handles through ARP and additional mechanisms. Important messages include Router Solicitation, Router Advertisement, Neighbour Solicitation, Neighbour Advertisement and Redirect.

Resolve link-layer addresses.
Discover routers and on-link prefixes.
Perform Duplicate Address Detection.
Track neighbour reachability.
Support Stateless Address Autoconfiguration (SLAAC).

Security/operations: Carelessly blocking ICMPv6 breaks fundamental IPv6 operation. Apply precise controls and use protections such as RA Guard where appropriate.

SLAAC, DHCPv6 and DNS

Router Advertisements tell hosts about prefixes and configuration flags. With SLAAC, a host forms an address from the advertised prefix and an interface identifier, then performs Duplicate Address Detection. Privacy extensions may rotate temporary interface identifiers.

DHCPv6 can be stateful (assigning addresses and other information) or stateless (providing options while SLAAC supplies the address). The default gateway is learned from Router Advertisements rather than from a DHCPv6 default-router option.

Configuration and verification

Cisco IOS-style example

ipv6 unicast-routing
!
interface GigabitEthernet0/0
 ipv6 address 2001:db8:10::1/64
 ipv6 enable
 no shutdown
!
ipv6 route ::/0 2001:db8:ffff::1

Useful checks

show ipv6 interface brief
show ipv6 interface
show ipv6 neighbors
show ipv6 route
ping ipv6 2001:db8:10::20
traceroute ipv6 2001:db8:20::20

Troubleshooting checklist

Check link-local and global addresses, prefix length and interface state.
Verify Router Advertisements and host configuration mode.
Inspect the IPv6 neighbour table and Duplicate Address Detection state.
Confirm the default route and source address selected by the host.
Permit essential ICMPv6 messages.
Check AAAA records and whether applications prefer an unreachable IPv6 path.
Test MTU and extension-header handling across security devices.
Verify dual-stack policy separately for IPv4 and IPv6; one does not automatically secure the other.

Interview focus

Use these points to practise concise, operations-focused answers. Explain the concept first, then give a verification or troubleshooting example.

How many bits are in an IPv6 address? 128 bits.
Can :: appear more than once in one IPv6 address? No, because expansion would become ambiguous.
What replaces ARP in IPv6? Neighbour Discovery using ICMPv6.
How does an IPv6 host normally learn its default gateway? From Router Advertisements.
Do IPv6 routers fragment transit packets? No. Fragmentation is performed by the source when needed.

Quick self-check

1. How many bits are in an IPv6 address?

Answer: 128 bits.

2. Can :: appear more than once in one IPv6 address?

Answer: No, because expansion would become ambiguous.

3. What replaces ARP in IPv6?

Answer: Neighbour Discovery using ICMPv6.

4. How does an IPv6 host normally learn its default gateway?

Answer: From Router Advertisements.

5. Do IPv6 routers fragment transit packets?

Answer: No. Fragmentation is performed by the source when needed.

Frequently asked questions

1. Does IPv6 use NAT by default?

Answer: No. End-to-end addressing is a primary design model, though translation mechanisms exist for specific transition or policy needs.

2. Is a link-local address optional?

Answer: IPv6 interfaces generally rely on link-local addressing for core local-link functions.

3. Why can IPv6 fail while IPv4 works?

Answer: It has separate addresses, routes, DNS records, firewall rules and neighbour-discovery dependencies.

Standards and further reading

Editorial note: Standards explain protocol behaviour, while commands and defaults can differ by vendor, platform and software release. Verify production changes against current vendor documentation.