Field Name

Size (bytes)

Description

Type

1

Type: Identifies the ICMPv6 message type; for Neighbor Advertisement messages the value is 136.

Code

1

Code: Not used; set to 0.

Checksum

2

Checksum: 16-bit checksum field for the ICMP header, as described in the topic on the ICMP common message format.

Flags

4

Target Address

16

Target Address: If the Neighbor Advertisement is being sent in response to a Neighbor Solicitation, this is the same value as in the Target Address field of the Solicitation. This field will commonly contain the IPv6 address of the device sending the Neighbor Advertisement, but not in all cases. For example, if a device responds as a proxy for the target of the Neighbor Solicitation, the Target Address field contains the address of the target, not the device sending the response. See the topic on address resolution proxying for details.

If the Neighbor Advertisement is being sent unsolicited, then this is the IPv6 address of the device sending it.

Options

Variable

Options: When sent in response to a multicast Neighbor Solicitation, a Neighbor Advertisement message must contain a Target Link-Layer Address option, which carries the link-layer address of the device sending the message. This is a good example of an “option” that's not really “optional”. J

When the Neighbor Advertisement is sent in response to a unicast Neighbor Solicitation, this option is technically not required (since the sender of the Solicitation must already have the target's link-layer address to have sent it unicast.) Despite this, it is still normally included, to ensure that the link-layer address of the target is refreshed in the cache of the device that sent the Neighbor Solicitation.