CCNA ? CCNP Training ? IPv6 Addressing
As a CCNA / CCNP candidate you are expected to understand IPv6
During your career as a Cisco network engineer you will have to deal understanding IPv6 address structure.
For your CCNA and CCNP studies you have to at some point confront and understand IPv6. At first glance it can see quite daunting compared to IPv4 that we are all used to, in actual fact IPv6 is quite a simply addressing protocol once you get past the initial shock. In this article we are going to have a look at Neighbour discovery protocol for layer 2 mapping.
When an IPv6 host or router needs to send a packet to some other host on the same network it will first of all look into it’s own local neighbour database to find if it has an IPv6 to MAC mapping, if it finds the right mapping the host will use it, if there is no mapping the host will need to resolve the known layer 3 IPv6 address to a currently unknown Layer 2 MAC address and to do this the host uses the Neighbor Discovery Protocol or NDP to discover the MAC address dynamically.
The Sending host will use a multicast message called a Neighbor Solicitation (NS) icmp message to ask the receiving host for it’s MAC address, the receiving host will reply with a Neighbor Advertisement (NA) icmp message unicast in return with the requested MAC address.
How IPv6 achieves this is all in the construction of the Neighbor Solicitation message which makes use of a special IPv6 destination address called a “Solicited Node Multicast”, this solicited Node Multicast at any given moment represented all of the IPv6 hosts on the link, the last 24 bits of the Solicited Node Address are the last 24 bits of the IPv6 address of the device that a host is requesting the MAC from.
The IPv6 multicast destination address is FF02::1:FF:0/104 the final 24 bits are made up of the last 24 bits of the IPv6 address to which the message is being sent to. For example if a host wanted to discover the MAC address of an IPv6 host addressed as 2222:3333:4444:5555:6666:AAAA:BBBB:CCCC:DDDD/64 then the solicited Node Address will look like the following FF02::1:FF:CC:DDDD/104
When a sending hosts wants to get the MAC address from the IPv6 host of 2222:3333:4444:5555:6666:AAAA:BBBB:CCCC:DDDD/64 it will take the last 24 bits of the known IPv6 address and place them into the remaining 24 bits of the Solicited Node Address and since all IPv6 hosts listen to their own Solicited Node addresses, when they hear their address they will reply with the MAC address.
Joe Spoto is a senior lecturer at Commsupport networks CCNA training
in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at Commsupport please visit us at CCNA Course
Commsupport run free one day training sessions and free on-line webinars, CCNA training