Neighbor Discovery Protocol

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Neighbor Discovery Protocol (NDP) is part of IPv6. It works at the internet layer and helps devices on the same local network communicate. Instead of using ARP like in IPv4, NDP uses ICMPv6 messages to handle essential tasks such as finding routers, learning addresses, and discovering gateways.

Five ICMPv6 message types help with these functions:
- Router Solicitation (RS)
- Router Advertisement (RA)
- Neighbor Solicitation (NS)
- Neighbor Advertisement (NA)
- Redirect

How it works (a simple example): Two computers, A and B, are on the same network. A wants to send a packet to B’s IPv6 address. A creates a solicited-node multicast address based on B’s address and sends a Neighbor Solicitation. B answers with a Neighbor Advertisement, sharing its MAC and IP addresses. A can then send the packet directly to B, and both A and B can cache the IP-to-MAC mapping to speed up future communications.

NDP also offers several improvements over IPv4:
- Neighbor Unreachability Detection (NUD): checks if a neighbor or link is still reachable, improving reliability.
- Inverse Neighbor Discovery (IND): maps a link-layer address back to an IPv6 address.
- Secure Neighbor Discovery (SEND): adds cryptographic security using Cryptographically Generated Addresses (CGA) and the Resource Public Key Infrastructure (RPKI).
- Neighbor Discovery Proxy (ND Proxy): helps bridge multiple network segments within a single subnet when link-layer bridging isn’t possible.

In short, NDP helps IPv6 devices find each other, resolve addresses, and stay connected reliably and securely.