What Unicast address scope operates within a network segment and does not require a router?

The link-local address scope is specifically designed for communication within a single network segment or local area network (LAN) without the need for a router. These addresses are automatically configured on devices when they connect to a network, allowing for immediate communication with other devices on the same segment.

Link-local addresses are typically used for purposes like address autoconfiguration and local network protocols, and they follow specific addressing formats depending on the protocol being used, such as IPv4 or IPv6. In IPv6, for example, link-local addresses start with the prefix "FE80::/10" and are not routable beyond their local segment, making them perfect for scenarios where routing is not required.

In contrast, the other address scopes listed do involve a broader range of functionality or require routing. Site-local addresses (prior to their deprecation) related to networks that can span multiple subnets but still required routing to communicate outside the local site. Aggregatable local addresses and aggregatable global addresses are meant for larger scales of networking and imply a hierarchical addressing structure that necessitates routing capabilities. Therefore, link-local addresses are the only type that strictly operates within a segment without any routing requirements.