computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

ing, a link-local address is a

unicast network address Unicast is data transmission from a single sender (red) to a single receiver (green). Other devices on the network (yellow) do not participate in the communication. In computer networking, unicast is a one-to-one transmission from one point in ...

that is valid only for communications within the

subnetwork A subnetwork or subnet is a logical subdivision of an IP network. Updated by RFC 6918. The practice of dividing a network into two or more networks is called subnetting. Computers that belong to the same subnet are addressed with an identica ...

that the host is connected to. Link-local addresses are most often assigned automatically with a process known as stateless address autoconfiguration or link-local address autoconfiguration, also known as automatic private IP addressing (APIPA) or auto-IP. Link-local addresses are not guaranteed to be unique beyond their network segment. Therefore, routers do not forward packets with link-local source or destination addresses.

IPv4 Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version d ...

link-local addresses are assigned from address block ( through ). In

IPv6 Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. I ...

, they are assigned from the block .

Address assignment

Link-local addresses may be assigned manually by an administrator or by automatic operating system procedures. In

Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. ...

(IP) networks, they are assigned most often using stateless address autoconfiguration, a process that often uses a

stochastic Stochastic (, ) refers to the property of being well described by a random probability distribution. Although stochasticity and randomness are distinct in that the former refers to a modeling approach and the latter refers to phenomena themselv ...

process to select the value of link-local addresses, assigning a pseudo-random address that is different for each session. However, in IPv6 the link-local address may be derived from the interface

media access control In IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublay ...

(MAC) address in a rule-based method. In IPv4, link-local addresses are normally only used when no external, stateful mechanism of address configuration exists, such as the

Dynamic Host Configuration Protocol The Dynamic Host Configuration Protocol (DHCP) is a network management protocol used on Internet Protocol (IP) networks for automatically assigning IP addresses and other communication parameters to devices connected to the network using a cli ...

(DHCP), or when another primary configuration method has failed. In IPv6, link-local addresses are always assigned, along with addresses of other scopes, and are required for the internal functioning of various protocol components. Updated by , , , , , .

IPv4

The

Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements an ...

(IETF) has reserved the IPv4 address block () for link-local addressing. The entire range may be used for this purpose, except for the first 256 and last 256 addresses ( and ), which are reserved for future use and must not be selected by a host using this dynamic configuration mechanism. Link-local addresses are assigned to interfaces by host-internal, i.e. stateless, address autoconfiguration when other means of address assignment are not available. The simultaneous use of IPv4 addresses of different scope on the same interface, such as configuring link-local addresses as well as globally routable addresses, may lead to confusion and increased complexity. Therefore, hosts search for a DHCP server on the network before assigning link-local addresses. In the automatic address configuration process, network hosts select a random candidate address within the reserved range and use

Address Resolution Protocol The Address Resolution Protocol (ARP) is a communication protocol used for discovering the link layer address, such as a MAC address, associated with a given internet layer address, typically an IPv4 address. This mapping is a critical functi ...

(ARP) probes to ascertain that the address is not in use on the network. If a reply is received to the ARP probe, it indicates the candidate IP address is already in use; a new random candidate IP address is then created and the process repeated. The process ends when there is no reply to the ARP, indicating the candidate IP address is available. When a globally routable or a

private address In Internet networking, a private network is a computer network that uses a private address space of IP addresses. These addresses are commonly used for local area networks (LANs) in residential, office, and enterprise environments. Both the I ...

becomes available after a link-local address has been assigned, the use of the new address should generally be preferred to the link-local address for new connections but communication via the link-local address is still possible.

Microsoft Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washi ...

refers to this address autoconfiguration method as Automatic Private IP Addressing (APIPA).

IPv6

In the Internet Protocol Version 6 (IPv6), the address block has been reserved for link-local unicast addressing. Of the 64 bits of a link-local addresses' network component, the most significant 10 bits (1111111010) correspond to the IANA-reserved "global routing prefix" for link-local addresses, while the "subnet ID" (the remaining 54 bits) is zero. Unlike IPv4, IPv6 requires a link-local address on every network interface on which the IPv6 protocol is enabled, even when routable addresses are also assigned. Consequently, IPv6 hosts usually have more than one

IPv6 address An Internet Protocol Version 6 address (IPv6 address) is a numeric label that is used to identify and locate a network interface of a computer or a network node participating in a computer network using IPv6. IP addresses are included in the p ...

assigned to each of their IPv6-enabled network interfaces. The link-local address is required for IPv6 sublayer operations of the

Neighbor Discovery Protocol The Neighbor Discovery Protocol (NDP), or simply Neighbor Discovery (ND), is a protocol of the Internet protocol suite used with Internet Protocol Version 6 (IPv6). It operates at the link layer of the Internet model, and is responsible for g ...

, as well as for some other IPv6-based protocols, such as

DHCPv6 The Dynamic Host Configuration Protocol version 6 (DHCPv6) is a network protocol for configuring Internet Protocol version 6 (IPv6) hosts with IP addresses, IP prefixes, default route, local segment MTU, and other configuration data required ...

. When using an IPv6 link-local address to connect to a host, a zone index must be added to the address so that the packets can be sent out on the correct interface. In IPv6, addresses may be assigned by stateless (automatic) or stateful (manual) mechanisms.

Stateless address autoconfiguration In computer networking, a link-local address is a unicast network address that is valid only for communications within the subnetwork that the host is connected to. Link-local addresses are most often assigned automatically with a process known as ...

is performed as a component of the Neighbor Discovery Protocol (NDP). The address is formed from its routing prefix and a unique identifier for the network interface. Through NDP routing prefix advertisements, a router or server host may announce configuration information to all link-attached interfaces which causes additional IP address assignment on the receiving interfaces for local or global routing purposes. This process is sometimes also considered stateless, as the prefix server does not receive or log any individual assignments to hosts. Uniqueness is guaranteed automatically by the address selection methodology. It may be MAC-address based, or randomized. Automatic duplicate address detection algorithms prevent assignment errors.

References

{{Reflist Computer configuration Network addressing

Address assignment

IPv4

IPv6

See also

References