Final answer:
The default router in an IPv6 network serves as the gateway for a host to access the global network, enabling communication with external networks. It does not provide link-local addresses or assign global addresses, which are managed through other protocols.
Step-by-step explanation:
In an IPv6 network, the role of the default router for a host in a routing domain is option B) It serves as the network's gateway to the global network. The default router is the device that enables communication between the local network and other networks, typically the internet. When a host wants to send data to a destination outside of its local network, it sends the traffic to the default router, which then routes the traffic onward.
The default router does not provide a host with a link-local address (which is auto-configured by the host itself) or assign unique global addresses to hosts; these are normally derived through mechanisms like Stateless Address Autoconfiguration (SLAAC) or Dynamic Host Configuration Protocol version 6 (DHCPv6). Nor is the default router solely for connecting hosts to a local network; that function is generally performed by switches or other networking devices within the LAN.