IPv6 Unicast Address Types: Global, Link-Local, and Unique Local

IPv6 unicast addresses identify a single interface. The main types are Global Unicast (publicly routable), Link-Local (required on every interface, only routable on the local link), and Unique Local (private, routable within an organization but not on the Internet).

Global Unicast Addresses (GUA)

A **Global Unicast Address (GUA)** is the IPv6 equivalent of a public IPv4 address. These addresses are globally unique and routable across the public Internet.

Currently, GUAs typically begin with a binary value of `001`, meaning they fall in the `2000::/3` range (which includes addresses starting with 2 or 3). A GUA usually consists of three parts: 1. **Global Routing Prefix:** Assigned by an ISP (often /48). 2. **Subnet ID:** Used by organizations to identify internal subnets. 3. **Interface ID:** The host portion, typically 64 bits.

Link-Local Addresses (LLA)

A **Link-Local Address (LLA)** is mandatory on every IPv6-enabled interface. These addresses are only valid and routable on the local network segment (the link). Routers will not forward packets with a link-local source or destination address.

LLAs always begin with `fe80::/10`. They are crucial for automatic address configuration (SLAAC), neighbor discovery (NDP), and routing protocol adjacencies (like OSPFv3). Even if a device has a GUA, it still uses its LLA for local subnet communication.

Unique Local Addresses (ULA)

**Unique Local Addresses (ULAs)** are roughly equivalent to IPv4 private addresses (RFC 1918). They are meant for local communications within a site or organization and are not routable on the global Internet.

ULAs fall within the `fc00::/7` block, though in practice, they are assigned from the `fd00::/8` block. They provide a way for organizations to assign IPv6 addresses without needing a prefix from an ISP, offering independence from external providers for internal traffic.

❓ Frequently Asked Questions