What is a link-local address and a global address in TCPIP V6?

When reading about routing in IP V6 there is frequent use of the terms link-local and global address. There are several definitions of what these mean – but I think you need to be an expert to understand them. Below are my definitions – they may be right – they may be wrong – but I hope they provide some useful information.

I found this site was a good basic guide to IP addresses.

Some basic information

  • A connection can either support IP V4 or IP V6 configuration
  • Each end of a connection has at least one IP address
  • IP addresses are hierarchical
    • The top 48 bits are global routing
      • If the top value is FE80 then the address is local. A router will not pass this outside.
      • If the top value is 2 then this is a global and the “ISP provider” is within the 48 bits. My Laptop has an IP V6 address like 2a00:1234:5678:9ABC. I used this site to display who my service provider was.
    • The next 16 bits are the subnet
    • The last 64 are the interface id
  • With IP V4, addresses starting 192.168 are local addresses and do not pass through a router – so FE80::… is not that new a concept.
  • A server needs fixed IP address, so clients can access it.
  • A client does not need a fixed IP address, as when it requests a service from the server, it sends its address as part of TCPIP. The server sends the reply back to this address.

The link local address

The link-local address at the end of a connection can have its address allocated:

  • Manually. This means you can give the IP address to your end users (or a DNS)
  • Allocated automatically. This can be based on the MAC address. IP V6 has network and neighbourhood discovery. As a connection becomes active it knows the IP address of the remote end, and can ask the remote end for the configuration it knows about. When the connection is closed, the network forgets the information about the connection, and its neighbourhood. The next time the connection starts, if it has a different IP address makes no difference – the information about the previous IP address has been forgotten
  • The advantage of having a different IP address is it obscures the client’s origin. If you have the same address each time, it is possible to build up a profile of your usage. If you use a different IP address – it makes it harder to do this.

If you have multiple network connections, you may need to help TCPIP send the traffic to the correct destination. For example

ping6 -I tap2 ….

which says sent it down the tap2 interface.

If you use ADCD the find_IO command gives

 FIND_IO for "colin@colin-ThinkCentre-M920s" 
         Interface IPv4    IPv6           
 Path    Name      Address Address        
------   --------- ---------------------- 
  A0     tap0 fe80::d442:b0ff:fe0c:96ab%tap0  
  A1     tap1 fe80::a015:53ff:fe0b:8685%tap1  

With FTP

ftp fe80::7:7:7:7%tap1

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