IPv6 Means Never Again Having to Wonder…

…about the IP address of your default router (default gateway in IPv4-speak).

It’s tough to argue against the fact that most IPv6 addresses are not much fun to type.  Being four times longer than IPv4 addresses and expressed in hexadecimal means things can get ugly on the keyboard pretty quickly.  For people in the IT field one very common mechanism for testing IP connectivity is to ping the default gateway.  And in IPv4 networks, the default gateway is always different for every layer-3 network.  It has now been a thousand bajillion times in my career when I have either asked someone or told someone what the default gateway is for a host who is having connectivity problems.  In IPv6 the ability (or inability) to ping the default router is just as helpful as it ever was in IPv4.  But there are a few apparent problems/challenges:

  1. IPv6 default router addresses can be painfully long.  Something like fe80::21a:a0ff:fe97:9ad3 is not unusual.  That’s a lot to type and if it fails you are always going to have to double-check to make sure you didn’t fat-finger the address (“Can I really not ping the gateway or is it that I just can’t type?)
  2. Assuming you have more than one interface in your device (an ethernet card and a WLAN card, for instance) you will need to specify the interface when pinging the link-local IPv6 address of the default router.  Because every interface has a link-local IPv6 address the system won’t know out which interface to send the packet unless you tell it.
    • For Windows:  ping fe80::21a:a0ff:fe97:9ad3%15 (where %15 identifies the interface number)
    • For Linux: ping -I eth0 -c 5 fe80::21a:a0ff:fe97:9ad3 will do it.

By all outward appearances the days of simply pinging your gateway (default) are gone.  But wait!  Not so!  What may appear to the first glances of many as an unappreciated addition of complexity can actually be much more simple than anything IPv4 could have offered.  Consider these facts:

  1. Unless otherwise configured most routers will automatically advertise themselves as a default router on each network segment they support.  This means that your devices never need to be configured with a default router; they learn it automatically by listening to the router’s advertisements.  Effort required by IT staff:  zero.
  2. Odd as it seems the IPv6 address of the default router is usually a link-local IPv6 address.  Link-local addresses are only relevant and useful on the local network segment (hence the name).  They have no meaning on other interfaces, even when those interfaces are on the same device.  This means that the link-local IPv6 address on interface fa0/1 of your router has nothing to do with the link-local IPv6 address on interface fa0/2 of the same router.  And this is true even though the addresses are technically on the same logical subnet (fe80::/10).  In IPv4 the router admins would be getting errors about overlapping networks but not so with IPv6.  The magic here is that the link-local IPv6 addresses on fa0/1 and fa0/2 are not overlapping or conflicting because they are not on the same network.  This means that they can even have the exact same IPv6 address and not conflict with each other!  That’s outright blasphemy in IPv4!  And this is exactly what I suggest you give some thought to doing:  make the link-local IPv6 address for every router interface in your whole internal network the exact same address (something simple, like fe80::1111 would do nicely).
  3. If every router interface has the same link-local IPv6 address the answer to the “what  is the default router’s address” question is never again going to be a mystery; it’s the same address for every single computer in your enterprise, no matter what network/VLAN they are currently connected to.

In the diagram below the two PC’s are on different physical networks (which translates to different logical layer-3 networks as well).  Both have a link-local IPv6 address that allows them to communicate with other nodes on their local LAN segment.  They cannot communicate with each other using these addresses.  They will need a Unique-Local or Global Unicast address if they want to exchange packets.  Each device has the same default router …or so it seems.  In actuality they both have a different default router, the address just happens to be the same.  The node on the left side of the diagram communicates with the link-local address configured on fa0/1 of the router.  The node on the right communicates with the link-local address configured on fa0/2 of the router.  The fact that both of those interfaces happen to have the same address is not relevant; the addresses are link-local.

Once you come to terms with functionality like this you begin to understand how IPv6 can take networking to new level while sometimes, just sometimes, making things more simple in the process.


Colin Weaver


About the Author

Colin Weaver

Colin Weaver is co-owner and lead instructor at ITdojo, Inc., a network security and information assurance training center and consulting firm located in Virginia Beach, VA. His passion for technology, networks, and security has led him to become enthralled with the idea of IPv6 and its implementation. In this blog he will share with you glimpses of what he has learned and a hint at what you’ll learn in his classes.