IPv6 Address Types and Notation
Key Takeaways
- IPv6 addresses are 128-bit values written in hexadecimal hextets separated by colons.
- Zero compression can be used once per address with double colon notation.
- Global unicast, unique local, link-local, multicast, loopback, and unspecified addresses have distinct purposes.
- IPv6 does not use broadcast; multicast and neighbor discovery replace many IPv4 broadcast behaviors.
- Link-local addresses commonly begin with fe80 and are used on the local segment, often with a zone or interface identifier.
IPv6 uses 128-bit addresses written as eight groups of four hexadecimal digits. Each group is a hextet. Hexadecimal digits are 0-9 and a-f.
| Full form | Compressed form | Notes |
|---|---|---|
| 2001:0db8:0000:0000:0000:0000:0000:0001 | 2001:db8::1 | Leading zeros removed, one zero run compressed |
| fe80:0000:0000:0000:0202:b3ff:fe1e:8329 | fe80::202:b3ff:fe1e:8329 | Link-local example |
| 0000:0000:0000:0000:0000:0000:0000:0001 | ::1 | Loopback |
| 0000:0000:0000:0000:0000:0000:0000:0000 | :: | Unspecified |
Compression Rules
- Remove leading zeros in each hextet.
- Replace one continuous run of all-zero hextets with ::.
- Use :: only once in an address.
- If there are multiple equal zero runs, compressing the leftmost run is a common convention.
| Invalid or risky notation | Why |
|---|---|
| 2001::db8::1 | Uses :: twice |
| 2001:db8:1 | Too few hextets without :: |
| 2001:db8:zzzz::1 | z is not hexadecimal |
IPv6 Address Types
| Type | Common prefix or address | Purpose |
|---|---|---|
| Global unicast | 2000::/3 | Internet-routable unicast |
| Unique local | fc00::/7, commonly fd00::/8 | Private internal addressing |
| Link-local | fe80::/10 | Local segment communication |
| Multicast | ff00::/8 | One-to-many groups |
| Loopback | ::1/128 | Local host testing |
| Unspecified | ::/128 | No address assigned or source before configuration |
| Documentation | 2001:db8::/32 | Examples and training |
Neighbor Discovery and No Broadcast
IPv6 does not use broadcast. Neighbor Discovery Protocol uses ICMPv6 messages for functions such as address resolution, router discovery, and duplicate address detection. Blocking all ICMPv6 can break IPv6 even when basic IPv4 habits suggest otherwise.
| IPv4 idea | IPv6 concept |
|---|---|
| ARP | Neighbor discovery |
| Broadcast | Multicast or neighbor discovery |
| DHCP default gateway option | Router advertisements provide default gateway information |
| 169.254.0.0/16 APIPA | fe80::/10 link-local always present on IPv6 interfaces |
PBQ-Style Thinking
Scenario: A host has only fe80:: addresses and cannot reach a remote subnet. That link-local address is normal, but it is not sufficient for routed global communication. Check router advertisements, DHCPv6 if used, prefix assignment, VLAN, and default route.
Scenario: A firewall blocks all ICMPv6 because the team treated it like optional ping traffic. IPv6 neighbor discovery and path MTU discovery can fail. The better control is to allow required ICMPv6 types and filter unsafe or unnecessary traffic according to policy.
Quick Recognition
| Address | Classification |
|---|---|
| 2001:db8::10 | Documentation prefix used in examples |
| 2606:4700:4700::1111 | Global unicast |
| fd12:3456:789a::1 | Unique local |
| fe80::1 | Link-local |
| ff02::1 | Multicast |
| ::1 | Loopback |
| :: | Unspecified |
Which IPv6 address is the loopback address?
An IPv6 address begins with fe80. What type of address is it most likely?
Which statements about IPv6 notation are correct? Choose two.
Select all that apply