IPv4 & IPv6 “Link-Local Addresses”

In a computer network a link-local address is a network address that is valid only for communications within the network segment (link) or the broadcast domain that the host is connected to.

Link-local addresses are not guaranteed to be unique beyond a single network segment. Routers therefore do not forward packets with link-local addresses. For protocols that have only link-local addresses, such as Ethernet, hardware addresses assigned by manufacturers in networking elements are unique, consisting of a vendor identification and a serial identifier.

IPv4 Link-local Addresses

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Link local addresses allow machines to automatically have an IP address on a network if they haven't been manually configured or automatically configured by a special server on the network (DHCP).The host can assign itself an IP address from a range of reserved Link-local addresses. Link local address ranges from 169.254.0.0 -- 169.254.255.255.

Assume a network segment where all systems are configured to acquire IP addresses from a DHCP server connected to the same network segment. If the DHCP server is not available, no host on the segment will be able to communicate to any other. Windows (98 or later), and Mac OS (8.0 or later) supports this functionality of self-configuration of Link-local IP address. In absence of DHCP server, every host machine randomly chooses an IP address from the above mentioned range and then checks to ascertain by means of ARP, if some other host also has not configured itself with the same IP address. Once all hosts are using link local addresses of same range, they can communicate with each other.

These IP addresses cannot help system to communicate when they do not belong to the same physical or logical segment. These IPs are also not routable.

Before an address is chosen from that range, the machine sends out a special message (using ARP which stands for address resolution protocol) to the machines on the network around it (assuming that they also haven't been assigned an address manually or automatically) to find out if 169.254.1.1 is free. If it is, then the machine assigns that address to its network card. If that address is already in use by another machine on the same network, then it tries the next IP 169.254.1.2 and so on, until it finds a free address.

The purpose of these self-assigned link-local addresses is to facilitate communication with other hosts within the subnet even in the absence of external address configuration (via manual input or DHCP). Unlike in IPv6, implementation of IPv4 link-local addresses is recommended only in the absence of a normal, routable address. Hosts pseudo-randomly generate the last two octets of the address to mitigate address conflicts. Because of the broadcast nature of some local networking protocols (for example, Microsoft's NetBIOS), hosts may be able to detect one another even without any preexisting knowledge of the address scheme.

IPv6 Link-Local Unicast Address:
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       A link-local unicast address is an IPv6 unicast address that is automatically configured on an IPv6 node interface by using the link-local prefix FE80::/10 (1111 1110 11) and the interface ID in the EUI-64 format.

        It is used to communicate with other nodes on the same link. The below figure shows two nodes on a single subnet using Link local IP addresses.  Two nodes on a same sub-net communicate using the Link local IP address ( No need of Routers)

                 

        

        NOTE: Routers will not forward any packets with link-local source or destination addresses to other links.

           

        

How to Use of EUI-64 Format in IPv6 Addresses in link local Address or Global?=================================================

EUI- Extended Universal Identifier

To create the IPv6 interface identifier from the 48-bit (6-byte) Ethernet MAC address:

The hexadecimal digits 0xFF-FE are inserted between the third and fourth bytes of the MAC address.

The Universal/Local bit (the second low-order bit of the first byte of the MAC address) is complemented. If it is a 1, it is set to 0; and if it is a 0, it is set to 1.

For example, for the MAC address of 00-60-08-52-F9-D8:

The hexadecimal digits 0xFF-FE are inserted between 0x08 (the third byte) and 0x52 (the fourth byte) of the MAC address, forming the 64-bit address of 00-60-08-FF-FE-52-F9-D8.

The Universal/Local bit, the second low-order bit of 0x00 (the first byte) of the MAC address, is complemented. The second low-order bit of 0x00 is 0 which, when complemented, becomes 1. The result is that for the first byte, 0x00 becomes 0x02.

As a result, the IPv6 interface identifier that corresponds to the Ethernet MAC address of 00-60-08-52-F9-D8 is 02-60-08-FF-FE-52-F9-D8.

The link-local address of a node is the combination of the prefix FE80::/64 and the 64-bit interface identifier expressed in colon-hexadecimal notation.

As a result, the link-local address of this example node, with the prefix of FE80::/64 and the interface identifier 02-60-08-FF-FE-52-F9-D8, is FE80::260:8FF:FE52:F9D8.

For Example:

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Uniqueness mask 000000X0 where X=1 is unique and X=0 in not unique. So if X=1 then the EUI-64 Address is 02 90 27 FF FE 17 FC 0F

NOTE :IMPORTANT
Characteristics:
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•  Mandatory addresses that are used exclusively for communication between  two IPv6 devices on the same link
•  Automatically assigned by device as soon as IPv6 is enabled
•  Not routable addresses (Their scope is link-specific only.)
•  Identified by the first 10 bits (FE80)
•  Typically created using the EUI-64 format

Addressing:
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•   Link Local Identifier (10 bits): Always begins with FE80::/10 (i.e. 1111 1110 10)
•  Remainder (54 bits): Could be all zeros or manually configured to another value.
•   Example: FE80:0000:0000:0000:0987:65FF:FE01:2345 or FE80::987:65FF:FE01:2345 (shorthand format)