What Is NDP?

Date First Published: 5th June 2022

Topic: Computer Networking

Subtopic: Internet Protocols

Article Type: Computer Terms & Definitions

Difficulty Level: 10/10

Learn more about what NDP is in this article.

Short for Neighbour Discovery Protocol, NDP is a protocol used to map an IP address to a fixed 48-bit MAC address in a LAN. It has a similar role to the Address Resolution Protocol (ARP). However, it translates 128-bit IPv6 addresses into MAC addresses rather than 32-bit IPv4 addresses.

It works between Layer 2 and Layer 3 of the OSI model. The MAC address works in the datalink layer to establish a connection between two devices in order for data transfer to occur and the IP address works in the network layer. NDP was introduced in March 1996 by the IETF as an extension to ARP. It was defined in RFC 1970, RFC 2461, and RFC 4861.

Note:

The Inverse Neighbour Discovery protocol is an extension to this protocol that enables nodes to determine and advertise an IPv6 address matching link-layer addresses, in a similar way to Reverse ARP for IPv4.

All determined addresses are stored in the neighbour cache, informing the network users about the local addresses of the neighbouring clients as well as additional information required for the availability check.

ICMPv6 Packet Types

Five ICMPv6 packet types are defined by NDP to perform functions for IPv6 in a similar way to ARP and ICMP Routing Discovery and Routing Redirect protocols for IPv4 for the purposes of router solicitation, router advertisement, neighbour solicitation, neighbour advertisement, and network redirects. These include:

• Router solicitation (Type 133) – These are messages that a host can send to request all routers of a network to send advertisements to the addressed router. These then respond with a type 134 message, which can be sent to the requesting host or to all subscribers of the network. This message type eliminates the need to wait for a router to automatically notify itself.
• Router advertisement (Type 134) – Occasionally, routers send router advertisements to inform network subscribers of their existence. Then, they distribute their routing information as well as the necessary factors for automatic IP configuration.
• Neighbour solicitation (Type 135) – Neighbour solicitations are sent by network subscribers to find the MAC address of a host. If these types of ICMPv6 messages want to determine an address, they will transmit information through multicast. If they want to check whether a neighbour exists, they will transmit it through unicast.
• Neighbour advertisement (Type 136) – These are used by node to respond to a neighbour's solicitation message. These messages may be unsolicited to quickly provide new information.
• Redirect (Type 137) – These are used by routers to inform hosts of a better first hop when on their way to a specific destination address. NDP-ICMPv6 redirects are sent to send these messages.