IPv6 Link Local

[Cisco Freak]

Hello All,

I was trying to learn the basics of IPv6.

C:\Users\User>ipconfig /all | Find "Link" 

   Link-local IPv6 Address . . . . . : fe80::f90f:542e:53ce:c7f1%13(Preferred) 

   Link-local IPv6 Address . . . . . : fe80::981a:7d25:c5c3:e277%12(Preferred)

While checking the Link local address , I see that the OS appends %13 and %12 at the end of the link-local address.

I have read that its related to multiple NICs in the machine.

My query is what exactly is the use if it?

CF

[Michael Vincent · Accepted solution]

Since all link local addresses start with fe80::/64, it's impossible to route for a machine with multiple interfaces - they all will have the same route.  The interface appended tells the operating system which interface to send the traffic out.  Note that once it's on the wire, it won't be routed off-net since it is link-local and no next-hop is defined.

See below, pinging (my defauilt gateway's link local address) without the interface results in destination unreachable, but appending the interface works:

----

VinsWorldcom@C:\Users\VinsWorldcom> ping fe80::5a6d:8fff:fe78:ad40

Pinging fe80::5a6d:8fff:fe78:ad40 with 32 bytes of data:
Destination host unreachable.
Destination host unreachable.

Ping statistics for fe80::5a6d:8fff:fe78:ad40:
Packets: Sent = 2, Received = 0, Lost = 2 (100% loss),
Approximate round trip times in milli-seconds:
Minimum = 3ms, Maximum = 3ms, Average = 3ms

VinsWorldcom@C:\Users\VinsWorldcom> ping fe80::5a6d:8fff:fe78:ad40%11

Pinging fe80::5a6d:8fff:fe78:ad40%11 with 32 bytes of data:
Reply from fe80::5a6d:8fff:fe78:ad40%11: time=40ms
Reply from fe80::5a6d:8fff:fe78:ad40%11: time=1ms

Ping statistics for fe80::5a6d:8fff:fe78:ad40%11:
Packets: Sent = 2, Received = 2, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 1ms, Maximum = 40ms, Average = 20ms

----

You can find the interfaces by using "netstat -rn"; the first number in the "Interface List" is the interface index number (the %# above):

----

VinsWorldcom@C:\Users\VinsWorldcom> netstat -rn
=================================================

Interface List

[...]

11...c0 cb 38 08 46 76 ......Dell Wireless 1397 WLAN Mini-Card
10...00 24 e8 9e f5 d7 ......Intel(R) 82567LM Gigabit Network Connection
[...]

1...........................Software Loopback Interface 1
=================================================

[...]

----

Or get more detail with:

wmic path Win32_NetworkAdapter where "MACAddress IS NOT NULL" get Index,NetConnectionID,Description,MACAddress,GUID /format:list

Cheers.

[Cisco Freak]

Thanks Michael!

[James Leinweber]

IPv4 and IPv6 are both packet-switched, next-hop routed, best-effort delivery protocols - by design - so at a WAN level they are fairly similar, except for the much bigger v6 addresses.  The biggest differences are actually at the LAN level, where v4 ARP got replaced by ICMPv6 Neighbor Discovery, ICMP Router Advertisements changed from an unused curiosity in v4 to a mandatory requirement in v6, and there is a new DHCPv6 protocol.  E.g. in v6 router address is not a DHCPv6 option, unlike v4; in v6 clients get the router address by scraping the link-local (!) router address off the router advertisement.   v6 has a rough equivalent of the v4 RFC-1918 private addresses in the v6 "unique local addresses" fc00::/7, but in practice those go unused; everyone is just using global scope 2000::/3 addresses.  And NAT66 pretty much doesn't exist.

-- Jim Leinweber, WI State Lab of Hygiene