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

Setting up IP V6 Linux to z/OS with ADCD

This post follows on from getting TCPIP to work with ADCD (zPDT and ZD&T) and allows me to FTP to z/OS from my Linux machine. There is a Q&A Has anyone configured z/OS running on ZD&T to support IPV6 protocol? but this was not enough information for me.

I’ve written about the IP V6 concepts and how they fit with z/OS.

You need to:

  1. Configure the Linux device map to add an OSA entry in the device map. You specify the path, and z/OS device addresses.
  2. Configure z/OS to support AF_NET6. You can make this change dynamically – or just re-ipl.
  3. Configure TCPIP to add an IFCONFIG6 entry. You need the z/OS AF_NET6 change before the IFCONFIG6 change is accepted. You need to restart TCPIP (or re-ipl).
  4. Configure a VTAM TRLE pointing to the devices defined in the devmap entry. This can be configured dynamically.
  5. Define a TCPIP interface, pointing to the PORTNAME of the TRLE definition. This defines a IP V6 address. The change can be configured dynamically.
  6. Test it!

Configuring the Linux Devmap

Create the Linux devmap entry

[manager]  # tap2 define network adapter (OSA) for communication with Linux
name awsosa 0019 --path=A2 --pathtype=OSD --tunnel_intf=y --tunnel_ip= 
  # QDIO mode
device 408 osa osa 
device 409 osa osa 
device 40a osa osa 

This session has IP V4 address, and uses device addresses 408,409 and 40a. It uses path A2.

Restart zD&T to pick up the changes, and re-ipl z/OS.

Configuring z/OS

You have to configure both z/OS and TCPIP to enable TCPIP V6 support.


Use D OMVS,S to show the BPXPRMxx members being used.

Update bpxprmxx with AF_INIT6, by adding the following into a BPXPRMxx member.


Check if your AF_INET is INET or CINET (Common INET is used when you have multiple TCPIP stacks), and specify the same value.


If you mis configure it

DOMAINNUMBER value 19 is required ( see DOMAINNUMBER ) When I used a different value I got


and, when TCPIP was started


Where 045A is EAFNOSUPPORT The address family is not supported.

Check AF_INET6 is configured

The command D OMVS,PFS gave me

 INET      EZBPFINI   N/A       SOCKETS   A     2022/09/20 04.08.00     
 NFS       GFSCINIT   NFSC      REMOTE    A     2022/09/20 04.07.23     
 ZFS       IOEFSCM    N/A       LOCAL     A     2022/09/20 04.07.19     
 AUTOMNT   BPXTAMD    N/A       LOCAL     A     2022/09/20 04.07.19     
 UDS       BPXTUINT   N/A       SOCKETS   A     2022/09/20 04.07.19     
PFS TYPE  DOMAIN        MAXSOCK  OPNSOCK  HIGHUSED                      
 INET     AF_INET6       50000        5         5                      
          AF_INET         64000        8         8                      
 UDS      AF_UNIX         10000        2         2                      

Check AF_INET6 is in the list.

Configure TCPIP

I added “include user.Z25A.tcpparms(iconfig6)” into the TCPIP PROF.

This member had just


Restart TCPIP.

The only change when TCPIP was restarted was the additional message


Check the configuration

On Linux the find_io command gave

FIND_IO for "colinpaice@colinpaice" 
         Interface Current     MAC     IPv4       IPv6           
 Path    Name      State       Address Address    Address        
------   --------- ----------- ------- ---------  ----------------  -------------- 
  A0     tap0      UP, RUNNING fa:...   fe80::f85c:c2ff:fe0a:1415%tap0  
  A1     tap1      UP, RUNNING 5e:... fe80::5cda:64ff:feee:eeaa%tap1  
  A2     tap2      UP, RUNNING 4a:... fe80::4850:5fff:fe5e:87c5%tap2 

Check the interface is UP, RUNNING

Define a VTAM TRLE

You need to create a VTAM TRLE resource. I invented PORTCP, and created member user.z25a.vtamlst(TRLE).

OSATRL3 VBUILD TYPE=TRL                                                 
               PORTNAME=PORTCP,                                        X

This uses address 0408,0409, and 040a (matching the devmap entry above)

Use V net,act,id=trle to activate it.

Note: USER.Z25A.VTAMLST is in the DD concatenation for //VTAMLST.

Use D NET,TRL to display the defined TRLs. This showed


Showing the TRLE above, and the status. It becomes ACTIVE when the TCPIP interface is activated.

Create the TCPIP interface definition

IP V6 uses an interface definition instead of a link and device.

    INTFID 7:7:7:7  
    IPADDR FD00::67:1:1 

I activated these using

  • v tcpip,,stop,ifportcp6
  • v tcpip,,obey,USER.Z25A.TCPPARMS(IFACE6)
  • You might need v tcpip,,stop,ifportcp6

I found it better to stop the interface before updating it, as sometimes the updates were not all made.

Once these definitions were activated, TSO NETSTAT HOME gave

IntfName:   IFPORTCP6
  Address:  fd00::67:1:1
    Type:   Global
  Address:  fe80::7:7:7:7
    Type:   Link_Local
    Flags:  Autoconfigured

This shows an address fd00::67:1:1 and address fe80::7:7:7:7 based on the INTFID. If you do not specify an INTFID you get a name like fe80::a2:a201:a2:a2a2, based on the chpid (value a2). If the chpid was changed, you would get a different IP address. You can see the chpid from the Linux from_io command, or the z/OS d NET,ID=OSATRL3E,E command.

I could not get any IP address specified in the IPADDR parameter, to work. I could ping to it, but there were no responses.

The interface gets a MAC address based on the CHPID value – for example MACADDRESS: 02A2A2A2A2A2.


IPV4 DESTINATIONS                                                     
DESTINATION        GATEWAY         FLAGS    REFCNT     INTERFACE         UH       0000000000 LOOPBACK         UH       0000000000 ETH1           H        0000000000 EZAZCX         H        0000000000 EZASAMEMVS     
IPV6 DESTINATIONS                                                     
DESTIP:   ::1/128                                                     
  GW:     ::                                                          
  INTF:   LOOPBACK6         REFCNT:  0000000000                       
  FLGS:   UH                MTU:     65535                            
DESTIP:   FD00::67:1/128                                              
  GW:     ::                                                          
  INTF:   IFPORTCP6         REFCNT:  0000000000                       
  FLGS:   UHS               MTU:     1492                             
DESTIP:   FD00::67:1:1/128                                            
  GW:     ::                                                          
  INTF:   IFPORTCP6         REFCNT:  0000000000    
  FLGS:   UH                MTU:     9000             
DESTIP:   FE80::7:7:7:7/128                   
  GW:     ::                                          
  INTF:   IFPORTCP6         REFCNT:  0000000000       
  FLGS:   UH                MTU:     9000                                

The Linux find_io command gave

      Interface  Current    MAC       IPv4        IPv6           
 Path Name       State      Address   Address     Address        
----- --------- ----------- --------  ----------  -------------- 
  A0 tap0       UP, RUNNING da:...    fe80::...tap0  
  A1 tap1       UP, RUNNING 92:...  fe80::...%tap1  
  A2 tap2       UP, RUNNING 42:...  fe80::...%tap2  

Update the Linux route information

I did this to try to get the IPADDR to work. It did not work, and so is this is not needed.

sudo ip -6 route add fd00::6:1:1/128 dev tap2

Test it!

Use TSO NETSTAT HOME to find the IP V6 address. For example

Address: fe80::7:7:7:7, Type: Link_Local

On Linux use the find_io command to display information about the tunnels to z/OS. Find the tapn matching the chpid being used on z/OS.
Use the

ping fe80::7:7:7:7%tap2

command to send data to z/OS.

The response to the ping will be sent back down the connection the request arrived on.

You can use the tso netstat devlinks(intfname IFPORTCP6 command (where IFPORTCP6 is my interface) to display information about just the specified interface; for example Inbound packets, BytesIn, Outbound packets, BytesOut.

You can use

FTP fe80::7:7:7:7%tap2

then use

tso netstat conn (port 21

to see the connections.

You can use NETSTAT ND to display the neighbours. This gave me

Query Neighbor cache for fe80::6a:ffff:feaf:c0e4
LinkLayerAddr: 026AFFAFC0E4 State: Reachable
Type: Host AdvDfltRtr: No

The value fe80::6a:ffff:feaf:c0e4 matches up with the value from find_io on Linux, and

02:6a:ff:af:c0:e4 matches up with the MAC address.

IP V6 concepts and using IP V6 with ADCD

This post follows on from getting TCPIP to work with ADCD (zPDT and ZD&T) and allow me to FTP to z/OS from my Linux machine. There is a Q&A Has anyone configured z/OS running on ZD&T to support IPV6 protocol? but this was not enough information for me.


With IP V4 there is a limit of the number of IP addresses available. IP V6 has many addresses, and so this should not be a problem. There is no smooth migration from IP V4 to IPV6, it is more start with IP V4, run IP V4 and V6 at the same time, move stuff from IP V4 to IP V6, – and possibly (unlikely) run with just IP V6.

Wikipedia has many good articles

  • IP V6 in general
  • IP V4 uses addresses like IP V6 uses addresses like 2001:0db8:0000:0000:0000:ff00:0042:8329 (=2001:db8::ff00:42:8329). See here.
  • Each IP V6 has a local address (link-local) fe80::….
  • An IP V6 address can have :: to mean replace with as many zeros as needed to make this a valid IP V6 address. So 2001:0db8:0000:0000:0000:ff00:0042:8329 can be written 2001:db8::ff00:42:8329. You can only have one :: in a value.
  • TCPIP can support IPV4 and IP V6 at the same time (dual stack)
  • You can wrap an IP V4 address into an IP V6. For example ::ffff:
  • For security clients often get a “temporary” (or randomised) IP address instead of a hard coded address. This uses a randomiser function with a secret key. This IP address can expire, and a new(different) IP address obtained. This can make it hard(impossible) for a server to do a reverse DNS lookup. This temporary IP address is useful, as it means you cannot be tracked by your IP address.

Other information

  • Each IP node has a IP V4 address and an IP V6 address.
  • When defining connections between systems, it looks like you need at least one IP V4 route, and at least one IP V6 route – I could be wrong.
  • An IPV6 host usually has more than one IP address.

IPV6 has reserved IP ranges

  • 2001:db8::/32 Addresses used in documentation and example source code.
  • fe80::/10 are the link-local unicast [RFC4291] addresses. Addresses within this block should not appear on the public Internet. Your router should not externalise this.
  • fd00::/7 for private internets. These are the unique-local addresses [RFC4193]. Addresses within this block should not appear by default on the public Internet. This means you can use them within your organisation.
  • ffxx is for multicast to all links matching the address. For example ff02::5 is used by the dynamic routing protocol OSPF to say to all routers in the (local) network “hello – anyone there”.

Getting started

The end of each connection needs at least one IP address. If you have 5 connections, you will have at least 5 IP addresses

What is my IP address on Linux ?

You can use hostname -I

which gave me

You can also use ifconfig or ip addr show.

You can also use the z109x find_io command.

What is my IP address on z/OS?

You can use TSO NETSTAT HOME, or the operator command V TCPIP,,NETSTAT,HOME .

This gives information like

LinkName:   ETH1
    Flags:  Primary
IntfName:   IFPORTCP6
  Address:  fc00::67:1:1
    Type:   Global
  Address:  fe80::a1:a101:a1:a1a1
    Type:   Link_Local
    Flags:  Autoconfigured

This shows

  • an IP V4 address for use within a private network, for connection ETH1.
  • fc00::67:1:1 an IP V6, unique local address, for interface IFPORTCP6.
  • fe80::a1:a101:a1:a1a1a link-local address, for interface IFPORTCP6. The a1:a1… is based on the MAC address of the device. You can override this on z/OS by specifying the INTFID.
  • Note: If you do not specify the INTFID, it will default to the MAC address. If you reconfigure the system, you may get different MAC address, and so the IP address via this interface will change. By specifying the INTFID you can specify what the IP address for this interface, which will not change if the system is reconfigured.

What are the routes on my machine?

IP V4 On Linux ip route or ip -4 route


default via dev wlp4s0 proto dhcp metric 600 dev tap0 proto kernel scope link src dev wlp4s0 scope link metric 1000 dev tap2 proto kernel scope link src dev tap1 proto kernel scope link src via dev tap2 dev wlp4s0 proto kernel scope link src metric 600 

IP V6 on Linux ip -6 route gives

:1 dev lo proto kernel metric 256 pref medium
2a00:xxx:xxxx:xxxx::/64 dev wlp4s0 proto ra metric 600 pref medium
fe80::/64 dev tap0 proto kernel metric 256 pref medium
fe80::/64 dev tap1 proto kernel metric 256 pref medium
fe80::/64 dev tap2 proto kernel metric 256 pref medium
fe80::/64 dev wlp4s0 proto kernel metric 600 pref medium
default via fe80::966a:b0ff:fe85:54a7 dev wlp4s0 proto ra metric 20600 pref medium

On z/OS, TSO NETSTAT route or V tcpip,,netstat,route gives

IPv4 Destinations
Destination        Gateway         Flags    Refcnt     Interface
-----------        -------         -----    ------     ---------
Default        GS       0000000000 IFPORTCP         UH       0000000000 LOOPBACK         UH       0000000000 ETH1
IPv6 Destinations
DestIP:   ::1/128
  Gw:     ::
  Intf:   LOOPBACK6         Refcnt:  0000000000
  Flgs:   UH                MTU:     65535
DestIP:   fc00::67:1:1/128
  Gw:     ::
  Intf:   IFPORTCP6         Refcnt:  0000000000
  Flgs:   UH                MTU:     9000
DestIP:   fe80::a1:a101:a1:a1a1/128
  Gw:     ::
  Intf:   IFPORTCP6         Refcnt:  0000000000
  Flgs:   UH                MTU:     9000


FIND_IO for "colinpaice@colinpaice" 
      Interface  Current MAC      IPv4      IPv6           
 Path Name       State   Address  Address   Address        
----- ---------- ------- ------- ---------- -------------- 
  A0 tap0 UP, RUNNING    ea:...   fe80::e8e8:69ff:fe20:435b%tap0  
  A1 tap1 UP, RUNNING    22:... fe80::2090:14ff:fee0:5f20%tap1  
  A2 tap2 UP, RUNNING    22:... fe80::2047:afff:fef7:1caf%tap2  

Joining it all up

To FTP from Linux to z/OS, I use

ftp fe80::a1:a101:a1:a1a1%tap1

The fe80::a1:a101:a1:a1a1 came from the z/OS NETSTAT HOME, and is the z/OS end of the connection.

Using another interface (defined with the INTFID 7:7:7:7) I could use

ftp fe80::7:7:7:7%tap2

The ip -6 route command gave me

fe80::/64 dev tap0 proto kernel metric 256 pref medium
fe80::/64 dev tap1 proto kernel metric 256 pref medium
fe80::/64 dev tap2 proto kernel metric 256 pref medium
fe80::/64 dev wlp4s0 proto kernel metric 600 pref medium

So the request for FE80…. can be routed to any of these. I know that it was configured using tunnel interface tap1, so the address to use is fe80::a1:a101:a1:a1a1%tap1.

Once the connection to FTP was established, z/OS TSO NETSTAT CONN gave me

FTPD1    0000003D ESTBLSH                             
  LOCAL SOCKET:   FE80::A1:A101:A1:A1A1..21           
  FOREIGN SOCKET: FE80::2090:14FF:FEE0:5F20..42572    

The request is processed by z/OS address FE80::A1:A101:A1:A1A1 (port 21)

This ties up with DestIP: fe80::a1:a101:a1:a1a1/128 … Intf: IFPORTCP6 which shows the request came in on interface IFPORTCP6

The request came from FE80::2090:14FF:FEE0:5F20, which ties up with fe80::2090:14ff:fee0:5f20%tap1. The request came in over the tap1 interface.