Link-local address
Encyclopedia
A link-local address is an Internet Protocol address
that is intended only for communications within the segment of a local network (a link
) or a point-to-point connection that a host is connected to. Routers do not forward packets with link-local addresses.
Link-local addresses may be assigned manually by an administrator or by operating system procedures. Most often they are assigned using stateless address autoconfiguration. In IPv4
, they are normally only used to assign IP addresses to network interfaces when no external, stateful mechanism of address configuration exists, such as the Dynamic Host Configuration Protocol
(DHCP), or when another primary configuration method has failed. In IPv6
, link-local addresses are mandatory, and required for the internal functioning of various protocol components.
Link-local addresses for IPv4
are defined in the address block 169.254.0.0/16. In IPv6
, they are assigned with the fe80::/64 prefix.
RFC 3927 warns against the simultaneous use of addresses of different scope, such as configuring link-local addresses as well as globally routable addresses on the same host. 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
(ARP) probes to ascertain that the address is not in use on the network. If a reply is received to the ARP, 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
become 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 communications via the link-local address is still possible.
Microsoft
refers to this address autoconfiguration method as Automatic Private IP Addressing (APIPA). It is sometimes also casually referred to as auto-IP.
(IPv6) requires a link-local address to be assigned to every network interface on which the IPv6 protocol is enabled, even when routable addresses are also assigned. A link-local unicast address has the prefix fe80::/64 in standard IPv6 CIDR notation
, even though fe80::/10 is allocated to it, because 54 bits are set to zero.
IPv6 hosts usually have more than one IPv6 address
assigned to each of their network interfaces. The link-local address is required for IPv6 sublayer operations of the Neighbor Discovery Protocol
. Link-local addresses may be assigned by automatic (stateless) or stateful (DHCPv6
) mechanisms.
In IPv6 stateless address autoconfiguration is performed as a component of the Neighbor Discovery Protocol
(NDP), as specified in RFC 4862. The address is formed from its routing prefix and the MAC address
of the interface.
Link-local IPv6 address assignment automatically implies the on-link presence for this routing prefix, unlike for the addresses of other scopes.
IPv6 introduced additional means of assigning addresses to host interfaces. Through NDP routing prefix advertisements, a router or a dedicated 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 MAC-address based methodology and duplicate address detection algorithms.
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
that is intended only for communications within the segment of a local network (a link
Link (telecommunications)
In telecommunications a link is the communications channel that connects two or more communicating devices. This link may be an actual physical link or it may be a logical link that uses one or more actual physical links...
) or a point-to-point connection that a host is connected to. Routers do not forward packets with link-local addresses.
Link-local addresses may be assigned manually by an administrator or by operating system procedures. Most often they are assigned using stateless address autoconfiguration. In IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
, they are normally only used to assign IP addresses to network interfaces when no external, stateful mechanism of address configuration exists, such as the Dynamic Host Configuration Protocol
Dynamic Host Configuration Protocol
The Dynamic Host Configuration Protocol is a network configuration protocol for hosts on Internet Protocol networks. Computers that are connected to IP networks must be configured before they can communicate with other hosts. The most essential information needed is an IP address, and a default...
(DHCP), or when another primary configuration method has failed. In IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
, link-local addresses are mandatory, and required for the internal functioning of various protocol components.
Link-local addresses for IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
are defined in the address block 169.254.0.0/16. In IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
, they are assigned with the fe80::/64 prefix.
IPv4
In RFC 3927, the Internet Engineering Task Force has reserved the address block 169.254.1.0 through 169.254.254.255The first and last 256 addresses in the 169.254/16 block are reserved for future use by RFC 3927. for link-local addressing in Internet Protocol Version 4. They are assigned to interfaces by host-internal, i.e. stateless, address autoconfiguration when other means of address assignment are not available.RFC 3927 warns against the simultaneous use of addresses of different scope, such as configuring link-local addresses as well as globally routable addresses on the same host. 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
Address Resolution Protocol
Address Resolution Protocol is a telecommunications protocol used for resolution of network layer addresses into link layer addresses, a critical function in multiple-access networks. ARP was defined by RFC 826 in 1982. It is Internet Standard STD 37...
(ARP) probes to ascertain that the address is not in use on the network. If a reply is received to the ARP, 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
Private network
In the Internet addressing architecture, a private network is a network that uses private IP address space, following the standards set by RFC 1918 and RFC 4193. These addresses are commonly used for home, office, and enterprise local area networks , when globally routable addresses are not...
become 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 communications via the link-local address is still possible.
Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
refers to this address autoconfiguration method as Automatic Private IP Addressing (APIPA). It is sometimes also casually referred to as auto-IP.
IPv6
Internet Protocol Version 6IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
(IPv6) requires a link-local address to be assigned to every network interface on which the IPv6 protocol is enabled, even when routable addresses are also assigned. A link-local unicast address has the prefix fe80::/64 in standard IPv6 CIDR notation
CIDR notation
CIDR notation is a compact specification of an Internet Protocol address and its associated routing prefix. Classless Inter-Domain Routing is an Internet Protocol address allocation and route aggregation methodology used within the Internet addressing architecture that replaced the IPv4 classful...
, even though fe80::/10 is allocated to it, because 54 bits are set to zero.
IPv6 hosts usually have more than one IPv6 address
IPv6 address
An Internet Protocol Version 6 address is a numerical label that is used to identify a network interface of a computer or other network node participating in an IPv6-enabled computer network....
assigned to each of their network interfaces. The link-local address is required for IPv6 sublayer operations of the Neighbor Discovery Protocol
Neighbor Discovery Protocol
The Neighbor Discovery Protocol is a protocol in the Internet Protocol Suite used with Internet Protocol Version 6 . It operates in the Link Layer of the Internet model and is responsible for address autoconfiguration of nodes, discovery of other nodes on the link, determining the Link Layer...
. Link-local addresses may be assigned by automatic (stateless) or stateful (DHCPv6
DHCPv6
DHCPv6 is the Dynamic Host Configuration Protocol for IPv6. Although IPv6's stateless address autoconfiguration can also be used to acquire IPv6 access, DHCPv6 may be a more suitable solution to assign addresses, nameservers and other configuration information as being done today with DHCP for IPv4...
) mechanisms.
In IPv6 stateless address autoconfiguration is performed as a component of the Neighbor Discovery Protocol
Neighbor Discovery Protocol
The Neighbor Discovery Protocol is a protocol in the Internet Protocol Suite used with Internet Protocol Version 6 . It operates in the Link Layer of the Internet model and is responsible for address autoconfiguration of nodes, discovery of other nodes on the link, determining the Link Layer...
(NDP), as specified in RFC 4862. The address is formed from its routing prefix and the MAC address
MAC address
A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
of the interface.
Link-local IPv6 address assignment automatically implies the on-link presence for this routing prefix, unlike for the addresses of other scopes.
IPv6 introduced additional means of assigning addresses to host interfaces. Through NDP routing prefix advertisements, a router or a dedicated 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 MAC-address based methodology and duplicate address detection algorithms.