IPv6 Address Structure
128-bit addresses written as 8 groups of 16-bit hex
IPv6 addresses are 128 bits long, written as 8 groups of 4 hex digits separated by colons. Two abbreviation rules apply:
Rule 1 — Omit leading zeros:
Rule 2 — Replace one longest run of all-zero groups with
0DB8 → DB8, 0001 → 1Rule 2 — Replace one longest run of all-zero groups with
:: (only once per address)
Full → Compressed Example
Full: 2001:0DB8:0000:0000:0000:0000:0000:0001
Compressed: 2001:DB8::1
Structure of a Global Unicast Address (GUA)
2001
:
DB8
:
ACAD
:
0001
:
AA
:
BB
:
CC
:
DD
◀──── Global Routing Prefix /48 ────▶
Sub
◀── Interface ID /64 ──▶
IPv6 Address Types
Know these for the exam
| Type | Prefix | Purpose |
|---|---|---|
| Global Unicast | 2000::/3 | Routable — public internet |
| Link-Local | FE80::/10 | Same link only, auto-configured |
| Unique Local | FC00::/7 | Private, not routed (like RFC 1918) |
| Multicast | FF00::/8 | One-to-many (no broadcast in IPv6) |
| Loopback | ::1 | Local loopback (equivalent to 127.0.0.1) |
| Unspecified | :: | Unknown source (DAD / DHCP discover) |
No broadcast in IPv6. Replaced by multicast. All-nodes multicast =
FF02::1. All-routers = FF02::2.
EUI-64 Interface ID
Derive a 64-bit Interface ID from a 48-bit MAC address
EUI-64 is used by SLAAC to auto-generate the host portion of an IPv6 address. Steps: split MAC in half, insert FF:FE in the middle, then flip bit 7 of the first byte.
Example: MAC = AA:BB:CC:DD:EE:FF
MAC OUI
AA:BB:CCMAC NIC
DD:EE:FF↓ Split + insert FF:FE + flip bit 7
Bit 7 flipped
A8BB
CC
Inserted
FFInserted
FEDD
EE
FF
Interface ID: A8BB:CCFF:FEDD:EEFF
Link-Local: FE80::A8BB:CCFF:FEDD:EEFF
Bit 7 flip: AA = 10101010 → bit 7 (U/L bit) flipped → 10101000 = A8. If bit 7 was 0 (globally unique), it becomes 1 (locally administered) and vice versa.
SLAAC & DHCPv6
How IPv6 hosts get their addresses
METHOD 1 — SLAAC (Stateless)
Stateless Address Autoconfiguration
Host sends RS → Router sends RA with prefix. Host generates own Interface ID (EUI-64 or random). No server needed. M=0, O=0 in RA flags.
METHOD 2 — Stateless DHCPv6
SLAAC + DHCPv6 for extras
Host gets prefix from RA (SLAAC), but uses DHCPv6 server for DNS/domain. M=0, O=1 in RA flags.
METHOD 3 — Stateful DHCPv6
Full DHCPv6 (like DHCPv4)
DHCPv6 server assigns full address + options. M=1 in RA flags. Server tracks leases.
Neighbor Discovery Protocol (NDP)
IPv6 replacement for ARP — uses ICMPv6 multicast
| Message | Type | Purpose |
|---|---|---|
| Router Solicitation (RS) | ICMPv6 133 | Host requests RA from routers |
| Router Advertisement (RA) | ICMPv6 134 | Router sends prefix + flags |
| Neighbor Solicitation (NS) | ICMPv6 135 | Find MAC for IPv6 (like ARP request) |
| Neighbor Advertisement (NA) | ICMPv6 136 | Reply with MAC (like ARP reply) |
| Redirect | ICMPv6 137 | Better next-hop exists |
DAD (Duplicate Address Detection): Before using an address, a host sends a Neighbor Solicitation for that address. If no NA comes back, the address is unique and safe to use.
Solicited-node multicastFF02::1:FFxx:xxxx
All-nodes multicastFF02::1
All-routers multicastFF02::2
All OSPF routersFF02::5
All DHCPv6 serversFF02::1:2
IPv4 vs IPv6 — Full Comparison
Key differences every CCNA candidate must know
| Feature | IPv4 | IPv6 |
|---|---|---|
| Address length | 32 bits (4 octets) | 128 bits (8 groups of 16 bits) |
| Address notation | Dotted decimal: 192.168.1.1 | Hexadecimal with colons: 2001:db8::1 |
| Address space | ~4.3 billion addresses | 340 undecillion (3.4 x 10^38) |
| Broadcast | Yes (255.255.255.255) | No — replaced by multicast and anycast |
| NAT required | Usually (private addressing) | No — every device gets global address |
| Header size | 20–60 bytes (variable with options) | 40 bytes fixed (extension headers separate) |
| Fragmentation | Routers and hosts | Source host only (Path MTU Discovery) |
| ICMP | ICMPv4 | ICMPv6 (includes ARP replacement — NDP) |
| Address config | Static or DHCP | Static, SLAAC, Stateless/Stateful DHCPv6 |
| ARP | ARP (broadcast-based) | NDP Neighbor Solicitation (multicast) |
| Checksum | Header checksum field | No header checksum (handled by L4/L2) |
| IPsec | Optional | Built into the protocol (mandatory support) |
IPv6 Address Compression Rules
Two rules for shortening IPv6 addresses
IPv6 addresses can be compressed using two rules. Both can be applied together. Knowing how to expand a compressed address is tested directly in the exam.
Rule 1 — Leading zeros: Remove leading zeros in each 16-bit group.
0DB8 → DB8 | 0001 → 1 | 0000 → 0
Rule 2 — Double colon (::): Replace one contiguous sequence of all-zero groups with
::. Can only be used once per address.
Full2001:0DB8:0000:0000:0000:0000:0000:0001
Rule 1 applied2001:DB8:0:0:0:0:0:1
Both rules2001:DB8::1
Ambiguous?FE80::1:0:0:1 — :: replaces the longest run; tie = leftmost
Loopback0:0:0:0:0:0:0:1 → ::1
Unspecified0:0:0:0:0:0:0:0 → ::
NDP — Neighbor Discovery Protocol
ICMPv6 messages replacing ARP and more
NDP is the IPv6 equivalent of ARP plus router discovery. It uses ICMPv6 messages sent to multicast addresses rather than broadcasts, making it more efficient and harder to spoof.
| Message | Type | Purpose | Sent to |
|---|---|---|---|
| RS — Router Solicitation | 133 | Host asks routers to identify themselves | FF02::2 (all routers) |
| RA — Router Advertisement | 134 | Router announces prefix, gateway info, flags | FF02::1 (all nodes) |
| NS — Neighbor Solicitation | 135 | Resolves IPv6 address to MAC (like ARP Request) | Solicited-node multicast |
| NA — Neighbor Advertisement | 136 | Replies with MAC address (like ARP Reply) | Unicast to requester |
| Redirect | 137 | Router tells host of a better next-hop | Unicast |
DAD — Duplicate Address Detection: Before using a new IPv6 address, a host sends an NS for that address. If no NA reply arrives, the address is unique and safe to use.
Stateful vs Stateless DHCPv6
RA flags determine which method is used
The RA message contains two flags — M (Managed) and O (Other) — that tell clients how to obtain their address and other config.
| M Flag | O Flag | Method | Address from | DNS/options from |
|---|---|---|---|---|
| 0 | 0 | SLAAC only | Self-generated (prefix + EUI-64/random) | RA |
| 0 | 1 | SLAAC + Stateless DHCPv6 | Self-generated | DHCPv6 server |
| 1 | 0/1 | Stateful DHCPv6 | DHCPv6 server assigns address | DHCPv6 server |
Set RA Flags on Router
! Stateless DHCPv6 (SLAAC + DNS via DHCPv6)
R1(config-if)# ipv6 nd other-config-flag
! Stateful DHCPv6 (DHCPv6 assigns address)
R1(config-if)# ipv6 nd managed-config-flag
IPv6 Unicast Types & Transition
Anycast, multicast, and IPv4/IPv6 coexistence
IPv6 has three address types — unicast (one-to-one), multicast (one-to-many), and anycast (one-to-nearest). There is no broadcast. Three main strategies exist for IPv4/IPv6 coexistence.
GUA (Global Unicast)2000::/3 — routable on the internet
Link-localFE80::/10 — required on every interface, not routed
Unique localFC00::/7 — private, not routed (like RFC 1918)
AnycastSame address on multiple devices — routed to nearest
MulticastFF00::/8 — one-to-many, no broadcast
Transition Strategies
Dual-stackDevice runs both IPv4 and IPv6 simultaneously
TunnellingIPv6 packets encapsulated inside IPv4 (6to4, GRE)
Translation (NAT64)Translates between IPv4 and IPv6 packet headers
IPv6 Routing Commands
R1(config)# ipv6 unicast-routing
R1# show ipv6 route
R1# show ipv6 interface brief
R1# show ipv6 neighbors ! NDP table (replaces show arp)
! OSPFv3 for IPv6
R1(config)# ipv6 router ospf 1
R1(config-rtr)# router-id 1.1.1.1
R1(config-if)# ipv6 ospf 1 area 0
IPv6 Knowledge Drills
Spaced repetition · 60-second timer per question
0
Correct
0
Wrong
0
Streak 🔥
60s
QUESTION 1 · IPv6
Packet Tracer Labs
Step-by-step IPv6 configuration walkthroughs.
IPv6 Topology Diagrams
IPv6 Cheatsheet
IPv6 Interface Configuration
# Enable IPv6 routing
R1(config)# ipv6 unicast-routing
# Manual GUA address
R1(config)# interface GigabitEthernet0/0/0
R1(config-if)# ipv6 address 2001:DB8:ACAD:1::1/64
R1(config-if)# no shutdown
# EUI-64 auto Interface ID
R1(config-if)# ipv6 address 2001:DB8:ACAD:1::/64 eui-64
# Manual link-local (optional)
R1(config-if)# ipv6 address FE80::1 link-local
# Verify
R1# show ipv6 interface brief
R1# show ipv6 interface GigabitEthernet0/0/0
DHCPv6 Server Config
# Stateless DHCPv6 (M=0, O=1)
R1(config)# ipv6 dhcp pool STATELESS
R1(config-dhcpv6)# dns-server 2001:DB8::1
R1(config-dhcpv6)# domain-name ccna.local
R1(config-if)# ipv6 dhcp server STATELESS
R1(config-if)# ipv6 nd other-config-flag
# Stateful DHCPv6 (M=1)
R1(config)# ipv6 dhcp pool STATEFUL
R1(config-dhcpv6)# address prefix 2001:DB8:ACAD:1::/64
R1(config-dhcpv6)# dns-server 2001:DB8::1
R1(config-if)# ipv6 dhcp server STATEFUL
R1(config-if)# ipv6 nd managed-config-flag
# Verify
R1# show ipv6 dhcp binding
IPv6 Static Routing
# Static route syntax
R1(config)# ipv6 route 2001:DB8:ACAD:2::/64 GigabitEthernet0/0/1
R1(config)# ipv6 route 2001:DB8:ACAD:2::/64 FE80::2
# Default IPv6 route
R1(config)# ipv6 route ::/0 2001:DB8:FEED::1
# OSPFv3 for IPv6
R1(config)# ipv6 router ospf 1
R1(config-rtr)# router-id 1.1.1.1
R1(config-if)# ipv6 ospf 1 area 0
# Verify routing
R1# show ipv6 route
R1# ping ipv6 2001:DB8:ACAD:2::1
Key IPv6 Facts
Address length128 bits (vs IPv4 32 bits)
Notation8 × 16-bit hex groups
GUA prefix2000::/3 (starts 001)
Link-local prefixFE80::/10
Loopback::1
No broadcastsReplaced by multicast
No NAT neededEnough GUAs for every device
Header size40 bytes (fixed, simpler than IPv4)
ICMP versionICMPv6 (NDP, MLD built-in)
DHCPv6 multicastFF02::1:2
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