FE80::/10 addresses: What are they really for?

Answered Question
May 26th, 2009

I'm playing with IPv6, and I know that FE80::/10 are link-local addresses. I believe that means that they are only known on the links that they're attached to. (If I'm wrong, please correct me.) I'm running OSPF between two routers, and I'm noticing that the only addresses that get put into the routing table are global addresses 2000::/3, but NONE of the FE80:: addresses are put into the routing table. FEC0:: addresses are put in the routing table, but I thought these addresses are obsolete?

So, what are the FE80 addresses really good for, and in what situations?

Thanks!

John

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Correct Answer by Giuseppe Larosa about 4 years 10 months ago

Hello John,

site local are routable because there is a 16-bit subnet field that can be different on link to link allowing 65535 /64 prefixes.

unique local are similar use fc00::/7 but expands the available space.

Cisco routers still support site local there was a thread about this few days ago.

To give an idea ipv4 multicast 224.0.0.X are said link local: they cannot be forwarded by a router from an interface to another interface.

ipv6 link local are the unicast counterpart of the same:

on a given ipv6 link = L2 broadcast domain each ipv6 host can send and receive ipv6 packets using link local.

Think of them as ipv4 packets with TTL=1 they cannot go in another subnet.

Hope to help

Giuseppe

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Giuseppe Larosa Tue, 05/26/2009 - 12:07

Hello John,

the FE80::/10 are the link local addresses.

It is correct that you don't see them in the routing table: as explained in a previous thread actually it is a single IPv6 /64 subnet using eui-64 format what you get.

Link local are very important in IPv6:

all mechanisms of Neighbor discovery, stateless address configuration (to learn unicast aggregatable addresses, link local or the modern unique local RFC4193), duplicate address detection,

ipv6 IGP adjacencies use link local.

Link local are used as source or destination addresses for all these activities.

Without link local what could be a valid ipv6 address to send a Router Solicitation for a just powered up ipv6 host ?

A RS uses FF02::2 (all routers in subnet) destination and uses the link local address as source (after the check for address duplication is passed the link local can be used).

In some ways link locals are a known starting point to allow all the activities on the link without using any broadcast or the old ARP protocol:

because an ipv6 host can have multiple ipv6 addresses on a NIC, link local are there to allow a start.

There is no ipv4 equivalent of this because ipv4 doesn't support multiple ip addresses per interface (yes there are secondary ip addresses but it is not the same).

Hope to help

Giuseppe

John Blakley Tue, 05/26/2009 - 12:12

Giuseppe,

Did the FEC0::/10 addresses get replaced by anything, or are these still non-routable? It seems that everything I'm seeing references the global unicast addresses, and nothing about site-local addresses.

As far as the link-local, it would seem as though they are just local to the router and hosts would never be able to see their addresses either?

Thanks,

John

Correct Answer
Giuseppe Larosa Tue, 05/26/2009 - 12:22

Hello John,

site local are routable because there is a 16-bit subnet field that can be different on link to link allowing 65535 /64 prefixes.

unique local are similar use fc00::/7 but expands the available space.

Cisco routers still support site local there was a thread about this few days ago.

To give an idea ipv4 multicast 224.0.0.X are said link local: they cannot be forwarded by a router from an interface to another interface.

ipv6 link local are the unicast counterpart of the same:

on a given ipv6 link = L2 broadcast domain each ipv6 host can send and receive ipv6 packets using link local.

Think of them as ipv4 packets with TTL=1 they cannot go in another subnet.

Hope to help

Giuseppe

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Posted May 26, 2009 at 11:35 AM
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