IPv6 Unique Local/Link-Local Detector

The IPv6 Unique Local/Link-Local Detector identifies the scope and type of any IPv6 address -- distinguishing between global unicast, unique local (ULA), link-local, multicast, loopback, and other special-purpose ranges. Enter an address like fe80::1, fd00::1, or 2001:db8::1 to see its classification, scope, and where it is valid on the network.

Understanding IPv6 Address Scopes

IPv6 was designed from the ground up with the concept of address scope -- the idea that different addresses have different reach. In IPv4, you have private addresses and public addresses, and that is essentially it. IPv6 introduces a richer model where addresses can be valid on a single network link only, within an organization, or across the entire internet. Getting the scope right is critical, because using the wrong type of address for a given purpose causes connectivity failures that can be confusing to diagnose.

The most important scope distinction in day-to-day IPv6 operations is between link-local and global addresses. Every IPv6-enabled interface automatically generates a link-local address in the fe80::/10 range without any configuration. This address exists only on the local network segment and is used for essential protocol operations like neighbor discovery and router solicitation. You cannot route traffic to a link-local address beyond the physical or virtual link where it was created. In contrast, a global unicast address from the 2000::/3 range is routable across the entire internet and is what you assign to servers and workstations that need external connectivity.

Between these two extremes sits the unique local address (ULA) space in the fc00::/7 range, with fd00::/8 being the commonly used subset. ULA addresses are the IPv6 equivalent of RFC 1918 private addresses -- they can be routed within your organization across multiple subnets and sites, but they are not routable on the public internet. They are ideal for internal services like DNS servers, databases, or monitoring systems that should never be directly reachable from outside. Understanding these scopes and when to use each type is fundamental to correct IPv6 deployment, and this detector makes the classification instant.

How to Detect IPv6 Address Scope and Type

1. Enter any IPv6 address in standard or compressed notation (e.g., fe80::1, 2001:db8::1, fd12:3456::1)
2. The tool identifies the address type based on its prefix bits
3. View the classification: global unicast, link-local, unique local, multicast, loopback, or other
4. See the scope information indicating where the address is valid (link, site, organization, global)

IPv6 Address Type Detection Examples

When to Use the IPv6 Address Type Detector

• Network configuration verification -- confirm that interfaces have the correct address types assigned (link-local for neighbor discovery, global for internet reachability)
• Firewall policy enforcement -- ensure link-local and ULA addresses are not being forwarded beyond their intended scope
• Troubleshooting connectivity -- diagnose routing failures caused by using a link-local address where a global address is required, or vice versa
• IPv6 migration planning -- identify which addresses in your inventory are ULA (internal-only) versus globally routable
• Security auditing -- detect ULA or link-local addresses leaking into public DNS records or application logs
• Multicast scope analysis -- determine whether a multicast address targets the local link, a site, an organization, or the global internet

How IPv6 Address Types and Scopes Work

IPv6 defines address types by their high-order bits, and each type has a specific scope that determines where the address is valid and routable.

Link-Local Addresses (fe80::/10)

Every IPv6-enabled interface automatically generates a link-local address in the fe80::/10 range. These addresses:

• Are valid only on the local network segment (link)
• Cannot be forwarded by routers
• Are used for Neighbor Discovery Protocol (NDP), router solicitation, and DHCPv6
• Always exist alongside any global or ULA addresses on the interface

A typical link-local address looks like fe80::a1b2:c3ff:fed4:e5f6, where the interface identifier is often derived from the MAC address using Modified EUI-64.

Unique Local Addresses (fc00::/7)

ULA addresses are the IPv6 equivalent of RFC 1918 private addresses. In practice, the fd00::/8 subset is used:

fd + 40-bit random ID + subnet ID + interface ID
fd12:3456:789a:0001:0000:0000:0000:0001
|--|----------|------|-----------------|
 fd  Global ID  Subnet  Interface ID

• Routable within an organization but not on the public internet
• The 40-bit random global ID minimizes collision risk when organizations merge networks
• Suitable for internal services that should not be internet-accessible

Global Unicast Addresses (2000::/3)

Addresses beginning with binary 001 (hex 2000 through 3fff) are globally routable:

• Assigned by ISPs and Regional Internet Registries (RIRs)
• Typically structured as /48 for sites, /64 for subnets
• Common prefixes include 2001:, 2400:, 2600:, 2800:, 2a00:

Multicast Scopes (ff00::/8)

Multicast addresses begin with ff and include a scope indicator in the fourth nibble:

For example, ff02::1 is "all nodes" on the local link, while ff0e::1 targets all nodes globally.

Practical Advice for IPv6 Address Scoping

The most common mistake engineers make when deploying IPv6 is not understanding that link-local addresses are always present and always used for control-plane operations, regardless of what other addresses are configured. When you run show ipv6 neighbors on a router, you will see link-local addresses in the neighbor table. When you configure OSPFv3 or EIGRPv6, the routing protocol forms adjacencies using link-local addresses. This is by design -- it means the routing protocol continues to work even if the global address changes. Do not try to filter or remove link-local addresses; they are required for IPv6 to function.

When designing an address plan for an organization, the question of whether to use ULA addresses alongside global addresses comes up frequently. The safe approach is to assign global unicast addresses to everything that might need external connectivity, and add ULA addresses only to services that must remain internal. Using ULA-only addressing for an internal service ensures that even if routing is misconfigured, traffic for that service cannot leak to the internet. However, dual-addressing (both global and ULA on the same interface) adds complexity and requires careful DNS management to ensure internal clients resolve to ULA addresses while external clients get no response at all.

Multicast scope is another area that catches administrators off guard. An ff02:: address (link-local scope) will not be forwarded by routers, which is correct for things like neighbor discovery. But if you need multicast to reach all devices across your site, you need ff05:: (site-local scope), and if it needs to span your entire organization across WAN links, you need ff08:: (organization-local scope). Misconfiguring the scope nibble means your multicast traffic either does not propagate far enough or propagates too far.

Frequently Asked Questions

What is the difference between link-local and unique local addresses in IPv6?

Link-local addresses (fe80::/10) are confined to a single network segment and cannot be routed at all. Unique local addresses (fd00::/8) can be routed within your organization's network across multiple subnets and sites, but are not routable on the public internet. Think of link-local as "this wire only" and ULA as "our company only."

Do I need both link-local and global addresses on an interface?

Yes. Every IPv6 interface must have a link-local address -- it is required for IPv6 to function because NDP and router discovery depend on it. A global unicast address is added when the interface needs internet reachability. Most production interfaces have at least one of each.

Why does my server log show fe80:: addresses even though I configured global addresses?

IPv6 neighbor discovery, router advertisements, and some local protocols always use link-local addresses. This is normal behavior. If an application is binding to fe80:: instead of your global address, check its listen configuration to ensure it uses the correct address or binds to all addresses (::).

Should I use ULA addresses for my internal services?

ULA addresses are a good choice for services that should never be directly accessible from the internet, such as internal DNS resolvers, database servers, and monitoring infrastructure. They provide stable addressing that does not depend on your ISP assignment. However, many organizations simply use their globally-routed prefix for everything and rely on firewall rules to restrict access, which is also a valid approach.

How do I tell the scope of a multicast address?

The scope is encoded in the fourth nibble (the second hex digit) of the multicast address. For example, ff02::1 has scope 2 (link-local), meaning it reaches only the local network segment. ff05::1 has scope 5 (site-local), and ff0e::1 has scope e (global). The scope determines how far routers will forward the multicast packets.

What is the fc00::/7 vs fd00::/8 distinction for ULA?

The fc00::/7 range covers all unique local addresses. Within that, fd00::/8 is for locally-assigned addresses where the 40-bit global ID is randomly generated. The fc00::/8 subset was originally intended for centrally-assigned ULAs, but no assignment mechanism was ever defined, so fd00::/8 is the only subset used in practice.

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