IPv6 is the next generation of IP, designed to replace IPv4 because IPv4 ran out of addresses and needs workarounds like NAT.

👉 IPv6 provides enough addresses for the whole planet, better routing and simpler network design.

Basic facts

• Address length: 128 bits
• Notation: hexadecimal (0–9, a–f)
• Example: 2001:0db8:abcd:0012:0000:0000:0000:0001 → shortened 2001:db8:abcd:12::1
• OSI layer: Layer 3
• Broadcast: ❌ does not exist in IPv6 · Multicast: ✅ used instead

Address structure

| Network/Prefix (routing) | Interface ID (host) |

Typical: 2001:db8:1000:1::/64 — /64 is the standard subnet (very important): first 64 bits = network, last 64 bits = host.

⚠️ SLAAC only works with /64 (classic exam trap).

IPv6 address types (the important ones)

Global Unicast

• Globally routable — IPv6’s answer to public IPv4 · prefix 2000::/3 · used on the internet · the most common type · hierarchical (ISP → site → subnet → interface), supports summarization.

Link-Local

• Automatically generated (mandatory) · prefix FE80::/10 · valid only on the local link · not routable
• Used by: Neighbor Discovery, routing protocols (OSPFv3, EIGRPv6, BGP next-hop)
• ⚠️ OSPFv3 adjacencies require link-local; routing cannot work without it

Unique Local (ULA)

• Private IPv6 · prefix FC00::/7 (in practice FD00::/8) · not routed on the internet
• IPv6’s answer to 10.0.0.0/8 / 192.168.0.0/16

Multicast

• One-to-many · prefix FF00::/8 · scope field defines reach · replaces broadcast
• Key addresses: FF02::1 all nodes, FF02::2 all routers

How does a device get an IPv6 address?

SLAAC (Stateless Address Autoconfiguration)

• The router sends Router Advertisements (RA); the client learns the prefix and generates its own interface ID.
• ✅ No DHCP server needed · ❌ no central control

DHCPv6

• Stateless (extra info only) or stateful (full address assignment)
• CCNP: understand the interplay between RA and DHCPv6

EUI-64 (automatic interface ID)

1. The MAC address is split in two
2. FFFE is inserted in the middle
3. The U/L bit (7th bit) is inverted

Status: still in the books, less used in practice (privacy extensions preferred) — but still exam material.

IPv6 subnetting (very simple)

Subnetting happens on the prefix, never on the host part:

2001:db8:1000::/48
 → 2001:db8:1000:1::/64
 → 2001:db8:1000:2::/64

Special IPv6 addresses

Address	Meaning
::	Unspecified
::1	Loopback
2001:db8::/32	Documentation (never in production)
::ffff:x.x.x.x	IPv4-mapped

Why is IPv6 better than IPv4?

• No NAT needed · better routing and summarization · built-in IPsec support · simpler neighbor discovery · (effectively) endless address space

Classic exam traps ⚠️

• SLAAC + /56 → ❌ wrong · OSPFv3 without link-local → ❌ · “broadcast” in IPv6 → ❌ (doesn’t exist) · forgetting multicast scope → ❌

Exam tips ⚠️

• OSPFv3 / EIGRPv6 problems? → check link-local
• SLAAC not working? → check /64
• “no broadcast”? → think multicast
• ULA vs Global? → a design choice

30-second summary

• 128-bit, hierarchical, no broadcast · /64 is king · link-local is critical for routing · multicast replaces broadcast · SLAAC + DHCPv6 = address management

Level	Expectation
CCNA	Types, notation, SLAAC, subnetting
CCNP	Design, summarization, routing behaviour, troubleshooting