TopicIT Networking →
Simple definition
NDP provides the core neighbor-discovery functions used on IPv6 networks.
Technical definition
Neighbor Discovery Protocol (NDP), defined primarily by RFC 4861, uses ICMPv6. IPv6 nodes use it to discover neighbors on the same link, determine link-layer addresses, find routers, and maintain reachability information.
What is it used for?
Discover IPv6 routers and neighbors, resolve a local IPv6 address to a link-layer address, and maintain the neighbor cache.
Practical example
An IPv6 host sends a Neighbor Solicitation to learn a neighbor’s link-layer address and receives a Neighbor Advertisement in response.
Common issues
- ICMPv6 blocked by a firewall
- Router Advertisements missing or filtered
- Stale or inconsistent neighbor-cache entries
Key takeaway: blocking ICMPv6 without understanding NDP can break normal IPv6 operation.