IPv6 unique local addresses (ULA) — behaviour and scope Which statement correctly describes a unique local address (ULA) in IPv6 and its intended routing scope?

Introduction / Context:
IPv6 unique local addresses (ULAs) provide an address space for internal use within organizations, analogous to private address blocks in IPv4. Engineers use ULAs to number internal links and hosts without exposing them to the public Internet, while still enjoying globally unique assignments that reduce collision risk during mergers or VPN interconnects.

Given Data / Assumptions:

• ULA prefix range is typically FC00::/7, commonly FD00::/8 for locally assigned space.
• ULAs are unicast addresses with limited routing scope (internal only).
• No NAT66 or special exception routing is assumed.

Concept / Approach:
ULAs are intended for intra-organization communication and are not advertised on the public Internet. They are still unicast addresses (one-to-one delivery), but their reach is restricted by policy and design. The pseudo-random Global ID component in a ULA helps avoid collisions when two private networks interconnect, improving on IPv4 private space where overlap is common.

Step-by-Step Solution:
Identify that the question targets ULA behaviour, not global unicast or link-local.Recall that ULA is conceptually similar to IPv4 private ranges (for example, 10.0.0.0/8), meaning “not routed on the public Internet”.Select the option that states these addresses are not meant to be publicly routed.

Verification / Alternative check:
Examine typical provider BGP policies and IPv6 best practices that filter FC00::/7 from public advertisement. Packet captures on Internet edges will not show ULA prefixes in global routing tables.

Why Other Options Are Wrong:

• A: True for any unicast, but not specific to the ULA concept.
• B: Describes global unicast behaviour, not ULAs.
• D: Describes link-local style nonroutable semantics, but ULAs are routable internally across sites, just not on the public Internet.
• E: Loopback is ::1/128; ULAs are assignable to normal interfaces.

Common Pitfalls:
Confusing ULAs with link-local addresses (FE80::/10). Link-local addresses are strictly one-hop only; ULAs can route within private domains.

Final Answer:
These are like private addresses in IPv4 in that they are not meant to be routed on the public Internet.