Link-local address

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 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 link-local addresses are assigned from address block 169.254.0.0 (169.254.0.0 through 169.254.255.255). In IPv6, they are assigned from the block fe80::. Link-local addresses may be assigned manually by an administrator or by automatic operating system procedures. In Internet Protocol (IP) networks, they are assigned most often using stateless address autoconfiguration, a process that often uses a stochastic 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 (MAC) address in a rule-based method, although this is deprecated for privacy and security reasons. 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 (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. The Internet Engineering Task Force (IETF) has reserved the IPv4 address block 169.254.0.0 (169.254.0.0 - 169.254.255.255) for link-local addressing. The entire range may be used for this purpose, except for the first 256 and last 256 addresses (169.254.0.0 and 169.254.255.0), which are reserved for future use and must not be selected by a host using this dynamic configuration mechanism.

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