Once the link local address is calculated, a host running IPv6 can use Stateless Address Autoconfiguration (SLAAC) to calculate a globally routable address. To calculate a global address using SLAAC,The IPv6 software sends a Router Solicitation (RS) message to a special router-only multicast address.

A router on the link will respond with a Router Advertisement (RA). The RA contains the segment prefix.

The IPv6 software combines the EUI-64 address, calculated above, with the segment prefix.

It performs duplicate address detection with this new address to make certain no other host on the segment has the same address.

It sets the default gateway address on the local host to the RA message source.

Note

If the host does not receive an RA in response to its RS message, there are no routers on the segment; hence, there is no way to communicate with any hosts on some larger network or the global Internet. If there are no routers, the host can assume every other host it can reach will be reachable using the link local address.

SLAAC is widely but not always used for configuring IPv6 interface addresses. Operators often note three problems with SLAAC:
•Naming services cannot be configured through SLAAC.
•It is difficult to relate an IPv6 address to an individual user or host in network management systems.
•SLAAC reveals potentially private information in some situations.

It is worth looking at the last item in this list in a little more detail. You can find more details at: https://www.acedexam.com/010-151-dctech-supporting-cisco-datacenter-networking-devices/

Physical Addresses and Privacy

Suppose you have a host—a laptop, mobile phone, or some other device—you use in several locations; Figure 3-2 illustrates.

Figure 3-2 Privacy and Host Movement

In Figure 3-2, a host moves from a home network at 1 to a coffee shop at 2. If the host uses SLAAC to calculate an IP address at both locations, server A can tell this is the same host in two locations because the lower 64 bits of the address will be the same.

In fact, no matter where you take this host, if you attach it to the global Internet and access this same server, server A will be able to know this is the same host. It is possible to track an individual user by noting their IPv6 address everywhere they go. This is a violation of the user’s privacy.

One solution to this problem is configuring the host to use a random physical address. Chapter 11, “Local Area Networks,” discusses this solution in more detail.

A second solution is to assign hosts addresses rather than calculating the interface address from the physical address.