Link Local Address

Link-local addresses are the IP addresses defined by the Internet Engineering Task Force (IETF) in RFC 3927. It is used for communication within a logical network segment and is not routed beyond that segment. They are automatically configured on interfaces for network devices to enable basic connectivity between devices on the same network, even if no external routing or global address configuration (such as DHCP or static IP) is available.

• Local Scope: Link-local addresses are only valid within the network segment (or link) where the device is connected. They cannot be routed across different networks or segments, meaning they cannot be used for communication between devices on different subnets or networks.

• Automatic Configuration: Devices can automatically assign themselves link-local addresses (such as with APIPA in IPv4 or SLAAC (Stateless Address Autoconfiguration) in IPv6), which enables basic communication even in the absence of a DHCP server.

• No Routing: Since link-local addresses are meant for local communication only, they are not routable. Devices will only communicate with other devices on the same physical or logical network segment using these addresses.

Address Format

IPv4 Link-Local:

169.254.x.x

IPv6 Link-Local:

fe80::/10 (often with the fe80 prefix, followed by a 64-bit interface identifier)

Example:

• If you have two devices (e.g., computers) on the same local network segment, they can communicate using IPv4 link-local addresses (e.g., 169.254.0.10 and 169.254.0.20), even if they do not have access to a DHCP server or external router.
• In IPv6, devices would automatically generate link-local addresses (e.g., fe80::abcd:1234:5678:9abc) and can use them to communicate with each other locally.

Common Use Cases

• IPv6 Neighbor Discovery: IPv6 link-local addresses are used for neighbor discovery and other protocol operations, even before global IPv6 addresses are assigned.
• Local communication: Devices on the same local network segment (like two computers in a home or office network) can communicate using link-local addresses without needing a router or a global IP address.
• Auto-configuration: When no external IP address assignment mechanism (like DHCP) is available, devices can use link-local addresses for basic networking.

Configurations of Link-Local Addresses in IPv4 and IPv6

1. IPv4 (Internet Protocol Version 4): The link-local addressing for IPv4 is specified in RFC (Request For Comments)3927 document by the IETF (Internet Engineering Task Force). The reserved range that belongs to link-local addresses is 169.254.0.0 - 169.254.255.255 by IETF. In case no option is available then interfaces got Link-local addresses through the internal host which implies stateless address autoconfiguration.

When the address is to be configured with automatic configuration then network hosts choose randomly candidate addresses and for that ARP (Address Resolution Protocol) is used. The purpose of using ARP is to confirm that the current address is not in use on the network. If a reply comes to ARP then it assures that candidate's IP address is not available to use because of its current usage. So, randomly a new candidate address is generated and the process is repeated. In other words, after the selection of an IP address, a query is sent by a link-local process with the same address on the network to check for its existence. In case of no response, there is an assignment of IP address to the device. Otherwise, the selection of another IP is made and the whole ARP process is repeated. 

When the assignment is done of link-local addresses and in case, there is the availability of globally routable addresses or private addresses than for new connections new addresses are preferred instead of available link-local addresses but communication can still be facilitated via link-local address. 

2. IPv6 (Internet Protocol Version 6): Every interface in IPv6 must have a link-local address. The link-local address is used between point-to-point interfaces as point-to-point communication is carried out between point-to-point interfaces that eliminate the need for a global IPv6 address. Furthermore, routers do not forward these addresses as they are used only on a single link. These are also used for neighbor discovery.

The following table shows the Link-local Unicast IPv6 address:

• The FE80 consists of 10 bits (1111111010) and the next 54 bits are 0's. 
• The prefix used with the IPv6 link-local address is FE80::/10.
• The address block termed as a prefix is reserved for the Link-local Unicast IPv6 address.

There are two possibilities for configuring IPv6 link-local addresses:

• Manually (stateful) 
• Auto address (stateless)

The Link-local prefix (FE80::/10) is used in automatic auto address configuration and Interface Identifier in the specified format of EUI-64. In the manual configuration method, the router command is used. The interface ID is obtained from the interface's hardware address or MAC address in EUI-64 Format.

In performing several sublayer operations of the Neighbor Discovery Protocol or other protocols such as DHCPv6 which are based on IPv6, this address is a must. As described in RFC 4862, the automatic configuration is implemented as part of the Neighbor Discovery Protocol (NDP) in IPv6. The address is made up of the routing prefix and the interface's MAC address.