The central problem
More addresses change operations.
Good network reasoning separates the mechanism from the symptom. Start with the boundary, identify the evidence, and test the explanation that accounts for the observation.
Find the local path
IPv6 uses local discovery before forwarding.
Neighbor Discovery Protocol (NDP) helps a host resolve a local neighbor, find a router, and learn whether an address is reachable on the local link. Router Advertisements can provide a prefix and guidance for address configuration.
These messages are local-link control traffic, not a replacement for end-to-end routing. A host can have a valid IPv6 address and still fail because discovery, router advertisements, filtering, or the return path is wrong.
Choose how a host is configured
IPv6 address assignment is not one single protocol.
SLAAC lets a host form an address from a prefix in a Router Advertisement. DHCPv6 can allocate addresses with leases or provide other configuration such as DNS resolver information. The Router Advertisement flags indicate whether the host should use managed address configuration or request other settings.
This is why a network can use SLAAC plus stateless DHCPv6, stateful DHCPv6, or a policy that relies mainly on Router Advertisements. The default route still comes from the router advertisement rather than from DHCPv6.
Keep an address while moving
Mobile IPv6 separates a stable home address from a changing point of attachment.
Mobile IPv6 lets a mobile node remain reachable through a permanent home address while it moves to another network. The node obtains a care-of address on the visited link and registers that binding with a home agent, which can tunnel traffic toward the current location.
Home address
The stable address applications and correspondents use to identify the mobile node.
Care-of address
The temporary address that describes the node's current point of attachment.
Home agent
Maintains the binding and forwards traffic when the node is away from its home network.
Route optimization
Can let a correspondent send more directly after authenticating the binding, avoiding triangular routing.
Mobile IPv6 is an IP-layer mobility mechanism, not a replacement for cellular radio mobility or an operator's core mobility system. Its security model must protect binding updates and prevent traffic redirection.
Build the model
Four useful signals
Address
Uses a 128-bit value and a prefix.
Neighbor Discovery
Finds local neighbors and routers.
Router Advertisement
Helps hosts learn prefixes and configuration.
Dual stack
Runs IPv4 and IPv6 during a transition.
Mini investigation
Inspect the evidence
Select the observation and identify the most direct next check.
This observation narrows the mechanism but does not prove every downstream layer.
Compare it with one nearby-layer check.
Knowledge check
Test your model
Answer five questions.