--- title: "Global unicast, link local, site local, multicast" subject: "Internet and Intranet" chapter: 2 key: "semester_8/internet_and_intranet:ipv6-address-types" source: https://www.bctengineer.com/q/semester_8/internet_and_intranet/ipv6-address-types site: BCT Engineering --- # Global unicast, link local, site local, multicast Internet and Intranet ยท Chapter 2: Internet Protocol Overview - Asked in: 2074 Magh, 10 marks - Video: [IPv6 address types | unicast | multicast | anycast | link local | ccna 200-301](https://www.youtube.com/watch?v=LU9Dz6WF40A) โ€” NETWORKING WITH H - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/internet_and_intranet/ipv6-address-types - Whole chapter: https://www.bctengineer.com/md/semester_8/internet_and_intranet/chapter-2 ## Question Explain Global Unicast, Link Local, Site local and Multicast address with examples and its scope. ## Short answer Global unicast (2000::/3) is globally routable, e.g. 2001:db8:1234:1::10. Link-local (fe80::/10) works only on one link, e.g. fe80::21a:2bff:fe3c:4d5e. Site-local (fec0::/10) is deprecated, replaced by unique local fc00::/7. Multicast (ff00::/8) delivers to a group, e.g. ff02::1. ## Answer IPv6 addresses are 128-bit. The kind of address and its scope are decided by the leading bits (prefix). | Type | Prefix | Scope | Example | |---|---|---|---| | Global unicast | 2000::/3 | Whole Internet | 2001:db8:1234:1::10 | | Link-local | fe80::/10 | Single link only | fe80::21a:2bff:fe3c:4d5e | | Site-local (deprecated) | fec0::/10 | One site | fec0:0:0:1::5 | | Multicast | ff00::/8 | Depends on scope field | ff02::1 | #### Global unicast Globally routable, unique one-to-one address, like a public IPv4 address. Structure: 48-bit global routing prefix (given by the ISP), 16-bit subnet ID, 64-bit interface ID. Example: `2001:db8:1234:1::10/64`. Scope: global; packets can cross the Internet. (2001:db8::/32 is reserved for documentation.) #### Link-local Automatically created on every IPv6 interface from prefix `fe80::/10` and the interface ID (usually EUI-64 or random). It is valid only on the local link, and routers never forward packets with link-local source or destination. Used for neighbour discovery, router advertisements and routing protocol next hops. Example: `fe80::21a:2bff:fe3c:4d5e`. Scope: one link/subnet. #### Site-local Prefix `fec0::/10`, meant for private addressing inside one site, similar to IPv4 private ranges (10.x). Because of ambiguity between sites it was **deprecated** by RFC 3879 and replaced by **Unique Local Addresses (fc00::/7, usually fd00::/8)**, for example `fd12:3456:789a:1::1`. Scope: one site or organisation, not routed on the global Internet. #### Multicast Identifies a group of interfaces; a packet is delivered to every member. It replaces broadcast. Format: `ff` + flags (4 bits) + **scope** (4 bits) + group ID. | Scope value | Scope | |---|---| | 1 | Interface-local | | 2 | Link-local | | 5 | Site-local | | 8 | Organisation-local | | e | Global | Examples: `ff02::1` all nodes on the link, `ff02::2` all routers on the link, `ff02::5` all OSPF routers, `ff05::2` all routers in the site, `ff0e::` global scope groups. #### Quick comparison of scope ```text Interface-local (::1) < Link-local (fe80::) < Site / Unique local (fd00::) < Global unicast (2xxx::) ``` - A link-local address is not unique across links; the same `fe80::1` may exist on many links, so the outgoing interface must be named (e.g. `fe80::1%eth0`). - Global unicast and unique local addresses are unique to a site or the world and are normally given to hosts by SLAAC, DHCPv6 or manual configuration. - Every IPv6 interface normally has several addresses at once: one link-local, one or more global, and the multicast groups `ff02::1` and the solicited-node group `ff02::1:ffxx:xxxx`. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is true about a link-local address (fe80::/10)? A. Routers forward it across the Internet B. It is valid only on a single link and is never forwarded by routers C. It is assigned only by DHCPv6 D. It is globally unique Answer: B. Link-local addresses are auto-created on every interface and are valid on one link only; they are used for neighbour discovery and routing next hops. (Key point: Link-local scope) 2. What replaced the deprecated site-local addresses (fec0::/10)? A. Global unicast 2000::/3 B. Link-local fe80::/10 C. Unique Local Addresses fc00::/7 (usually fd00::/8) D. Multicast ff00::/8 Answer: C. RFC 3879 deprecated site-local because of ambiguity between sites; Unique Local Addresses replaced them. (Key point: Site-local deprecation) 3. In an IPv6 multicast address ff02::1, what does the digit 2 represent? A. Group ID 2 B. Link-local scope C. Global scope D. Flags value for permanent group Answer: B. The format is ff + flags + scope + group ID; scope 2 is link-local, so ff02::1 means all nodes on the link. (Key point: Multicast scope) 4. Which describes the global unicast address 2001:db8:1234:1::10/64? A. Prefix 2000::/3, routable on the whole Internet, one-to-one B. Prefix fe80::/10, valid on one link C. Prefix ff00::/8, one-to-many D. Prefix fec0::/10, one site only Answer: A. Global unicast uses 2000::/3 and is like a public IPv4 address with global scope. (Key point: Global unicast) 5. Why must the outgoing interface be named with a link-local address, such as fe80::1%eth0? A. Because it is longer than other addresses B. Because the same fe80::1 may exist on many links C. Because link-local is routed globally D. Because it replaces the MAC address Answer: B. Link-local addresses are unique only within a link, so the interface identifies which link is meant. (Key point: Link-local ambiguity) ## Related in other subjects - [Multicast and unicast routing protocols](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Amulticast-unicast-routing) โ€” Computer Networks and Security, chapter 4 - [Global vs local; & and * operators](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Aglobal-local-ampersand-star) โ€” Computer Programming, chapter 3