18

What is the difference between link-local address and global link address in ISATAP?

For example:

link-local address: fe80::5efe:c000:0201
global address: 3ffe:b00:1:2::5efe:c000:0201

Why don't we use just one address? Why it is helpful to use both?

random
  • 450
  • 1
  • 9
  • 16
JoesyXHN
  • 293
  • 1
  • 2
  • 6

1 Answers1

24

Any IPv6 adapter should always have two IP addresses if you're using it for internet traffic - your link-local address and your global address.

Your global address is world-routable, so anyone anywhere in the world can see that IP address (although of course, there should be a firewall between you and them to stop them from actually accessing you).

Your link-local address is just for your local area network. Consider it the equivalent of a 192.168.0.1 or 10.1.1.1 address. They are not routable, and can be used for internal communications, so that if your world-routable prefix changes, you don't have to update all your IP references to internal IP addresses.

The only functional difference is that your link-local address is not routable, and your global address must be routable. There are issues where non-routable global addresses have been allocated (via DHCPv6) for whatever reason, your computer THINKS it has a routable IPv6 address when it doesn't, and then all your IPv6 connectivity breaks.

I also notice that your global IP address has been assigned manually with a dot-decimal address, which makes me wonder if your IPv6 network is configured correctly as it's obviously not using auto-config with RA announcements)

Mark Henderson
  • 68,316
  • 31
  • 175
  • 255
  • Hi, thanks for your concrete answer. This holds true for ISATAP. How about 6to4 tunneling? i see that there is only one address of IPv6 which is created based on ipv4 address? So I guess it is Ipv6 link-local address but then it is absent of ipv6 global address. Please correct me if I am wrong. Thanks in advance. –  Mar 02 '11 at 00:16
  • One more thing: fe80::a124:45f6:b8d7:cf7f%41 => this is a link local address I suppose? what does % mean? thanks –  Mar 02 '11 at 00:30
  • @Joe - the [% sign indicates which adapter the IP address is in](http://en.wikipedia.org/wiki/IPv6_address#Link-local_addresses_and_zone_indices). As far as 6to4, I've never really done much of it so I won't be much help Im afraid. – Mark Henderson Mar 02 '11 at 01:07
  • On the 6to4, the address generated is a /48, so you can subnet that into 65k different subnets, each with a full /64 of host addresses. – Chris S Mar 02 '11 at 20:20
  • Your comparison is misleading because RFC 1918 addresses are not link-local. The link-local addresses in IPv4 are 169.254.0.0/16 as defined in RFC 3927. – kasperd Jul 10 '15 at 07:15