In IPv6 addressing, a link-local address is always taken from which prefix at the start of the address?

Introduction / Context:
Every IPv6 enabled interface automatically configures a link-local address that is used for communication on the local link only. These addresses are critical for Neighbor Discovery, router advertisements, and many other basic IPv6 functions. Knowing the prefix that identifies link-local addresses is essential for troubleshooting and design.

Given Data / Assumptions:

• We are dealing with IPv6 link-local addresses, not global unicast or unique local addresses.
• Link-local addresses are valid only on a single Layer 2 segment and are never routed across routers.
• The question asks which prefix a link-local address always starts with.
• Several well known IPv6 prefixes are provided as options.

Concept / Approach:
IPv6 link-local addresses are defined in the FE80::/10 prefix. This means that any address whose high order bits match FE80 through FEBF belongs to the link-local range. In practice, most implementations use FE80::/64 for link-local address assignment, with the remaining bits often derived from the interface identifier. Other prefixes such as FC00::/7 and 2001:db8::/32 serve different purposes and are not used for link-local addressing.

Step-by-Step Solution:
1. Recall that link-local addresses are used for local link communication and start with FE80 in most configurations.2. Understand that the formal prefix is FE80::/10, which covers a small range of addresses beginning with FE8, FE9, FEA, and FEB.3. Recognize that many textbooks simplify this to FE80::/64 because that is the most common subnet size used in practice for link-local addresses.4. Compare this knowledge to the options.5. Option C correctly states that link-local addresses start with FE80::/10 and notes the typical FE80::/64 usage, so it is the best answer.

Verification / Alternative check:
On any IPv6 capable operating system, running a command to display interface addresses will show at least one address that begins with FE80 on each IPv6 enabled interface. Network references and standards documents also identify FE80::/10 as the reserved link-local range.

Why Other Options Are Wrong:
Option A, 2001:db8::, is reserved for documentation and examples in textbooks and is not link-local. Option B, FF00::, defines the IPv6 multicast address range, which is fundamentally different from link-local unicast addresses. Option D, FC00::/7, represents unique local addresses, which are similar to private addresses in IPv4 but are routable within an organization and not restricted to a single link.

Common Pitfalls:
Students sometimes confuse link-local addresses with unique local or global unicast addresses because they all may appear on the same interface. Another common mistake is believing that link-local addresses can be routed across subnets, when in fact they are strictly limited to the local link. Remember that FE80 at the start of an IPv6 address is a strong indicator that the address is link-local.

Final Answer:
Addresses starting with FE80::/10, commonly written as FE80::/64 for typical link-local use.