Link-Local vs. Global Unicast (IPv6) – Detailed Explanation

1. Definition and Scope

🔹 Link-Local Address

Valid only on a single physical/logical link.
Automatically assigned and mandatory for IPv6-enabled interfaces.
Used primarily for local communications, including neighbor discovery.

🔹 Global Unicast Address

Globally routable IPv6 address used for Internet-wide communication.
Equivalent to IPv4 public addresses.

2. Address Formats and Prefixes

Address Type	Prefix Range	Prefix Length	Notes
Link-Local	`FE80::/10`	10 bits	Starts from `FE80` to `FEBF`
Global Unicast	`2000::/3`	3 bits	Starts with `2` or `3`

3. Address Structure

Interface Identifier (IID): Usually derived automatically (EUI-64 or randomly generated).
IID uniquely identifies an interface on a given link.

4. Automatic Address Configuration

Link-Local: Automatically configured upon enabling IPv6 (no DHCP).
Global Unicast: Can use manual, SLAAC, or DHCPv6 for configuration.

5. Usage and Reachability

Address Type	Scope	Routing Behavior
Link-Local	Single Link (LAN)	Not forwarded by routers (local only)
Global Unicast	Global (Internet)	Forwarded globally by routers

6. Neighbor Discovery Protocol (NDP)

Uses Link-Local addresses for Neighbor Solicitation (NS), Neighbor Advertisement (NA), and router discovery.
Routers use link-local for device management and routing protocols locally.

7. Routing and Forwarding Behavior

Link-Local: Traffic remains within local link segment.
Global Unicast: Traffic routed across the IPv6 Internet.

8. Multicast and Anycast Interaction

Link-local utilizes solicited-node multicast addresses for neighbor discovery.
Global unicast can employ anycast for optimized routing.

9. Security Considerations

Use RA guard and ND inspection for securing link-local addresses.
Properly configure firewall and ACLs to filter global unicast addresses at network boundaries.

10. Practical Examples

Ping Link-Local Address:

ping6 fe80::1%eth0

Ping Global Unicast Address:

ping6 2001:db8::1

View IPv6 Addresses:

ip -6 addr show

11. Transition Mechanisms

Used in tunneling (6to4, ISATAP) for IPv6 encapsulation.
Critical for establishing tunnels and neighbor relationships.

12. Troubleshooting Techniques

Commands for troubleshooting IPv6:

show ipv6 neighbors
show ipv6 route

13. Best Practices

Use link-local addresses for local communication, NDP, and routing management.
Global unicast addressing should be carefully planned for scalability.
Regularly monitor and secure link-local and global unicast traffic.

✅ Summary Table

Aspect	Link-Local Address	Global Unicast Address
Prefix	`FE80::/10`	`2000::/3`
Scope	Local Link Only	Global (Internet)
Routable	No	Yes
Address Assignment	Automatic (SLAAC)	Manual, SLAAC, DHCPv6
Used For	NDP, Local Communication	Internet Communication
Packet Forwarding	Not Forwarded	Globally Forwarded

Link-Local vs. Global Unicast (IPv6) Quiz

1. What is a Link-Local IPv6 address?

A. An address used for Internet-wide communication B. An address manually configured for external routing C. An address valid only on a single physical or logical link D. An address used only for multicast communication

Correct answer is C. Link-Local IPv6 addresses are valid only on the local network segment and cannot be routed beyond it.

2. Which prefix range is reserved for IPv6 Link-Local addresses?

A. FE80::/10 B. 2000::/3 C. FC00::/7 D. FF00::/8

Correct answer is A. IPv6 Link-Local addresses use the FE80::/10 prefix range.

3. How are Global Unicast IPv6 addresses best described?

A. Local to one subnet only B. Used exclusively for multicast C. Manually assigned without routing capability D. Globally routable addresses used for Internet communication

Correct answer is D. Global Unicast addresses are globally unique and routable on the IPv6 Internet.

4. What prefix range is assigned for Global Unicast IPv6 addresses?

A. FE80::/10 B. 2000::/3 C. FC00::/7 D. FF00::/8

Correct answer is B. Global Unicast addresses start with the prefix 2000::/3.

5. Which method is commonly used to generate the Interface Identifier (IID) for Link-Local addresses?

A. EUI-64 method or random generation B. Manual configuration only C. DHCPv6 exclusively D. IPv4 address conversion

Correct answer is A. The IID for Link-Local addresses is usually generated automatically via the EUI-64 method or random assignment.

6. How are Link-Local IPv6 addresses assigned?

A. Manually only B. Only via DHCPv6 C. Through IPv4 DHCP D. Automatically when IPv6 is enabled, without DHCP

Correct answer is D. Link-Local addresses are assigned automatically on every IPv6-enabled interface without DHCP.

7. Which protocol uses Link-Local addresses to perform neighbor discovery and router communication?

A. DHCPv6 B. ICMPv4 C. Neighbor Discovery Protocol (NDP) D. DNS

Correct answer is C. NDP relies on Link-Local addresses for neighbor solicitation and advertisement messages.

8. Are Link-Local IPv6 packets forwarded by routers?

A. Yes, always B. No, only valid within the local link C. Only if configured manually D. Only for multicast traffic

Correct answer is B. Routers do not forward Link-Local traffic beyond the local segment.

9. What prefix do solicited-node multicast addresses, used in neighbor discovery, derive from?

A. Interface Identifier (IID) B. Global Unicast prefix C. Link-Local prefix D. Anycast address

Correct answer is A. Solicited-node multicast addresses are derived from the Interface Identifier portion of the IPv6 address.

10. Which address type is essential for establishing IPv6 tunnels like 6to4 and ISATAP?

A. Global Unicast address B. Multicast address C. Link-Local address D. Anycast address

Correct answer is C. Link-Local addresses are used in IPv6 tunneling protocols like 6to4 and ISATAP.

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