Cisco CCNA Preparation · lesson 34 of 36 · 25 min
Why IPv6?
IPv4 provides ~4.3 billion addresses — exhausted in 2011. IPv6 uses 128-bit addresses, providing 340 undecillion (3.4 × 10³⁸) addresses — enough for every grain of sand on Earth to have trillions of addresses.
IPv6 also eliminates the need for NAT (every device gets a globally unique address), simplifies header processing (fixed 40-byte header), and builds in features like stateless address autoconfiguration (SLAAC) and mandatory IPsec support.
IPv6 Address Format and Notation Rules
IPv6 addresses are 128 bits written as 8 groups of 4 hex digits separated by colons: 2001:0DB8:0000:0001:0000:0000:0000:0001
Abbreviation Rule 1 — Leading zeros: Drop leading zeros in each group. 0DB8 → DB8, 0001 → 1, 0000 → 0.
Abbreviation Rule 2 — Double colon (::): Replace one contiguous sequence of all-zero groups with ::. Can only be used ONCE per address. 2001:DB8:0:1:0:0:0:1 → 2001:DB8:0:1::1.
Prefix notation: Like CIDR for IPv4. 2001:DB8::/32 means the first 32 bits are the network prefix. Common prefixes: /64 (standard subnet), /48 (site), /32 (ISP allocation), /128 (single host).
Expanding :: Back
To expand ::, count existing groups, then fill with enough all-zero groups to make 8 total. 2001:DB8::1 has 3 groups (2001, DB8, 1) — expand to: 2001:0DB8:0000:0000:0000:0000:0000:0001.
IPv6 Address Types
Global Unicast (GUA) — routable on the internet. Starts with 2000::/3 (first hex digit 2 or 3). Equivalent to public IPv4 addresses.
Link-Local — automatically assigned to every IPv6-enabled interface. Starts with FE80::/10. Only valid on the local link — not routed. Used for neighbor discovery and routing protocol adjacencies.
Unique Local (ULA) — private addresses, not routed on internet. Starts with FC00::/7 (FC or FD). Equivalent to RFC 1918 private IPv4.
Multicast — starts with FF00::/8. Replaces IPv4 broadcast. Key multicast groups: FF02::1 (all nodes), FF02::2 (all routers), FF02::5 (OSPFv3 routers), FF02::6 (OSPFv3 DR/BDR).
Anycast — same address assigned to multiple interfaces; packets are delivered to the nearest one. Used for load balancing and redundancy.
Loopback — ::1/128 (equivalent to 127.0.0.1 in IPv4).
Unspecified — ::/128 — used as source address when a device has no address yet (DHCP discovery).
EUI-64 and SLAAC
EUI-64 — method to auto-generate the 64-bit interface ID from a 48-bit MAC address: split MAC in half, insert FFFE in the middle, flip the 7th bit (universal/local bit). Example: MAC 00:1A:2B:3C:4D:5E → EUI-64 021A:2BFF:FE3C:4D5E.
SLAAC (Stateless Address Autoconfiguration) — devices auto-configure their IPv6 address using the network prefix from a Router Advertisement (RA) + EUI-64 interface ID. No DHCP server needed.
DHCPv6 — stateful (assigns full address) or stateless (provides DNS/domain info only, address from SLAAC). Managed flag (M) and Other flag (O) in RA control which method is used.
Key Terms
Key Terms
Knowledge Check — 12 Questions
IPv6 Addressing
How many bits are in an IPv6 address?
- 32
- 64
- 128
- 256
Answer: 128
IPv6 addresses are 128 bits, written as 8 groups of 4 hex digits.
Which IPv6 address type starts with FE80::/10?
- Global Unicast
- Unique Local
- Link-Local
- Multicast
Answer: Link-Local
Link-local addresses (FE80::/10) are automatically assigned to every IPv6 interface and are only valid on the local link.
The double colon (::) in an IPv6 address represents:
- A single all-zero group
- One or more consecutive groups of all zeros (used once per address)
- The end of the address
- A multicast prefix
- Answer: One or more consecutive groups of all zeros (used once per address)
:: replaces one contiguous sequence of all-zero groups. It can only appear once in an address.
Which IPv6 address is the loopback address?
- ::0
- ::1
- FE80::1
- FF02::1
Answer: ::1
::1/128 is the IPv6 loopback address, equivalent to 127.0.0.1 in IPv4.
Global Unicast Addresses (GUAs) start with which prefix?
- FE80::/10
- FC00::/7
- 2000::/3
- FF00::/8
Answer: 2000::/3
GUAs start with 2000::/3 — the first hex digit is 2 or 3. These are internet-routable addresses.
IPv6 Unique Local Addresses are equivalent to which IPv4 address type?
- Public IP addresses
- RFC 1918 private addresses
- APIPA addresses
- Multicast addresses
Answer: RFC 1918 private addresses
Unique Local (FC00::/7) are private IPv6 addresses — not routed on the internet, equivalent to 10.x, 172.16.x, 192.168.x.
SLAAC allows devices to configure their IPv6 address without:
- A router
- A DHCP server
- A MAC address
- An RA message
- Answer: A DHCP server
SLAAC uses Router Advertisements (RA) to provide the network prefix — no DHCP server is required.
Which multicast address reaches all IPv6 routers on a link?
- FF02::1
- FF02::2
- FF02::5
- FF02::9
Answer: FF02::2
FF02::2 is the all-routers multicast address. FF02::1 reaches all nodes (hosts and routers).
The standard subnet prefix length for IPv6 is /_____.
Answer: 64
/64 is the standard IPv6 subnet size — the first 64 bits are the network prefix, the last 64 bits are the interface ID.
EUI-64 generates the interface ID by:
- Using the first 64 bits of the MAC address
- Splitting the 48-bit MAC, inserting FFFE, and flipping the 7th bit
- Hashing the MAC address with SHA-256
- Using a random 64-bit number
- Answer: Splitting the 48-bit MAC, inserting FFFE, and flipping the 7th bit
EUI-64: split MAC at byte 3, insert FF:FE, flip bit 7 (U/L bit) of the first byte.
IPv6 eliminates the need for NAT because every device can have a globally unique address.
- True
- False
Answer: True
With 128-bit addresses, every device can have a unique global address — NAT is no longer needed for address conservation.
Which abbreviated form of 2001:0DB8:0000:0000:0000:0000:0000:0001 is correct?
- 2001:DB8::1
- 2001:DB8:0::1
- 2001::DB8::1
- 2001:DB8:0:0:0:0:0:1
Answer: 2001:DB8::1
2001:DB8::1 — leading zeros dropped in each group, and the six consecutive zero groups replaced with ::.
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