How does link local address work ?

[dolanduck.]

How does the link local Fe80 work? i am reading odem Wendel official cert guild and I'm straight up confused  on how it routes packets with link local?

do host use the link-local address only when in there subnet ?

link-local packets are not routed outside the link or the local subnet right ?

but in the book in the picture example the host sends a packet to the link local address of a router?

why not use the ipv6 uni-cast address?

it used the link local wouldn't that screw up the destination ipv6 address of the packet ?

[Giuseppe Larosa]

Hello @dolanduck.  

Link local addresses are local to a single link they cannot be routed.

However, link local addresses are an easy way to build protocol neighborships like OSPFv3 adjacencies and what is more important they are a valid next-hop.

In the context of a specific link,  a link local address is unique and it can be used as a next-hop for unicast destinations

So it cannot be routed it cannot be a destination address going out of link, but it is a valid next-hop.

A good reason for using link local addresses to build protocol neighborships is that they will never change unless someone change the host portion of a link local address.

The unicast addresses that can be on-link , they can be changed by design using appropriate time values valid up to valid from time that are used in router advertisements.

In other words IPv6 has built in features for re-addresssing a site or enterprise network when needed and these procedures allow for smooth changes.

Seen in this bigger picture the choice to use link local addresses for routing protocols makes sense.

Hope to help

Giuseppe