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Link-Local Addresses
Also called Automatic Private IP Addressing (APIPA)
Link-Local Addresses are a type of IP address used for communication between devices on the same local network segment, or link. These addresses are automatically assigned by devices when they are unable to obtain an address from a DHCP server or when no other external configuration methods are available. Link-Local Addresses are primarily used for communication within a network where devices need to connect without requiring external routing or access to the internet. The related RFC is RFC 3927, which defines the use of IPv4 Link-Local Addresses. https://en.wikipedia.org/wiki/Link-local_address https://tools.ietf.org/html/rfc3927
In IPv4, Link-Local Addresses are assigned within the 169.254.0.0/16 address range, with the addresses ranging from 169.254.1.0 to 169.254.254.255. Devices that support Automatic Private IP Addressing (APIPA) will assign themselves an address from this range when no DHCP server is available. These addresses allow devices to communicate with each other on the same local network without requiring manual configuration. However, they cannot be used for communication across different subnets or to access external networks. The related RFC is RFC 2131, which defines how DHCP interacts with IPv4 address assignment. https://en.wikipedia.org/wiki/Automatic_Private_IP_Addressing https://tools.ietf.org/html/rfc2131
For IPv6, Link-Local Addresses play an essential role in network configuration and are automatically assigned to every IPv6-enabled interface. The IPv6 Link-Local Addresses begin with the prefix fe80::/10. These addresses are mandatory for every IPv6 device and are used for tasks such as neighbor discovery and router advertisement. Even in networks with full IPv6 connectivity, Link-Local Addresses remain crucial for local communications between devices. The related RFC is RFC 4862, which defines IPv6 address autoconfiguration and the assignment of Link-Local Addresses in IPv6 networks. https://en.wikipedia.org/wiki/IPv6_link-local_address https://tools.ietf.org/html/rfc4862
The primary function of Link-Local Addresses is to facilitate local communication between devices without the need for routing or external configuration. This is particularly useful in environments where no network infrastructure, such as routers or DHCP servers, is present. In such cases, devices can still communicate and share resources like printers or files within the same subnet. However, because Link-Local Addresses are non-routable, they are limited to local communication and cannot be used to send traffic beyond the immediate network segment. The related RFC is RFC 4291, which defines the IPv6 addressing architecture, including Link-Local Addresses. https://en.wikipedia.org/wiki/Link-local_address https://tools.ietf.org/html/rfc4291
Another significant feature of Link-Local Addresses is their role in network autoconfiguration. When a device is connected to a network, it can use Link-Local Addresses to perform tasks like address resolution and neighbor discovery without needing to communicate with a DHCP server or obtain a global IP address. This enables basic network functionality immediately upon connection, simplifying the process of connecting devices in dynamic or temporary environments. The related RFC is RFC 4861, which discusses the neighbor discovery process in IPv6 networks. https://en.wikipedia.org/wiki/Neighbor_Discovery_Protocol https://tools.ietf.org/html/rfc4861
Although Link-Local Addresses are useful for local network communication, they are not suitable for broader networking needs. For example, a device using a Link-Local Address cannot communicate with devices on other networks or access the internet. This limitation is why Link-Local Addresses are often seen as a fallback option when more formal network configurations, such as DHCP or static addressing, are unavailable. Despite this, Link-Local Addresses are integral to certain types of local communication, such as IPv6 autoconfiguration and peer-to-peer connections. The related RFC is RFC 3927, which explains the role of Link-Local Addresses in both IPv4 and IPv6 networks. https://en.wikipedia.org/wiki/IPv6_link-local_address https://tools.ietf.org/html/rfc3927
Link-Local Addresses also play a role in some specialized networking applications, such as Zero-configuration networking (Zeroconf). In these systems, devices automatically configure themselves and discover other devices on the same network without manual intervention. This is particularly common in home networks, small office networks, and IoT environments, where users may not have the technical expertise to manually configure network settings. Zeroconf relies on Link-Local Addresses to allow devices to communicate instantly after being connected to the network. The related RFC is RFC 6762, which defines Multicast DNS as part of Zero-configuration networking. https://en.wikipedia.org/wiki/Zero-configuration_networking https://tools.ietf.org/html/rfc6762
Conclusion
The title of this RFC is “Link-Local Addresses.” Link-Local Addresses provide an essential mechanism for local network communication without requiring external configuration or routing. They are used in both IPv4 and IPv6 networks to allow devices to communicate within the same local segment. Although they are limited to local communication and cannot be routed to other networks or the internet, Link-Local Addresses serve a critical role in autoconfiguration, neighbor discovery, and peer-to-peer networking. They are commonly used in smaller, unmanaged networks and environments where immediate, local communication is necessary without reliance on broader network infrastructure.