IPv6 Fundamentals

Learning objective

Read, classify, shorten, and troubleshoot IPv6 addresses, then explain how a host forms an address, discovers neighbours, and chooses local delivery or a router.

Why IPv6 exists

IPv6 provides a vastly larger address space than IPv4 and restores room for end-to-end addressing. It also replaces several familiar IPv4 mechanisms: hosts use Neighbor Discovery rather than ARP, routers advertise network information, and multicast replaces broadcast. Examples here use 2001:db8::/32, which is reserved for documentation and must not be treated as a deployable allocation.

Address structure and hexadecimal

An IPv6 address contains 128 bits written as eight 16-bit hextets. Each hexadecimal digit represents four bits, so one hextet contains four hexadecimal digits. Colons separate hextets; they do not mark packet or subnet boundaries by themselves.

Expand and shorten addresses safely

Leading zeroes may be removed from each hextet. One longest consecutive run of all-zero hextets may be replaced by ::; use it only once. Thus 2001:0db8:0000:0000:0000:0000:0000:0001 becomes 2001:db8::1. Expansion reverses both rules until exactly eight hextets are visible.

Interactive IPv6 address explorer

Use the player to unfold an address, identify its prefix and interface portions, and compare its type and scope. The visual conclusion always remains visible: shortening changes notation, never the 128-bit value.

Explore an IPv6 address

Choose an address type

Global documentation addressLink-local neighbourUnique-local serviceLink-local multicast groupLoopbackUnspecified source

Try an IPv6 address and prefixExplore address

2001:db8:0:0:20c:29ff:fe9c:409/64

Step 1 of 7: Validate the address

20010db800000000020c29fffe9c0409

2001:db8::20c:29ff:fe9c:409

64 network bits · 64 interface bits

global-unicast · global

Identifies one interface in globally routed IPv6 space.

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Prefixes and interface identifiers

A prefix length states how many leading bits identify the network prefix. /64 is common on SLAAC LANs, leaving 64 interface-identifier bits, but IPv6 supports other prefix lengths. A device compares the destination with its configured on-link prefixes before selecting a next hop.

Address types and scopes

• Global unicast addresses are globally scoped; this lesson represents them with documentation block 2001:db8::/32.
• Link-local addresses use fe80::/10 and remain on the local link.
• Unique-local addresses use fc00::/7 and are intended for private internal use, not public Internet routing.
• Multicast addresses use ff00::/8; their scope field limits where the group is meaningful.
• ::1 is loopback, while :: is the unspecified address and is not assigned to an interface.

IPv6 has no broadcast

IPv6 has no broadcast address. A sender uses a specific multicast group when multiple listeners must receive a packet. The Ethernet switch can still forward the multicast frame across eligible ports, but only nodes listening for that IPv6 group accept and process it.

Neighbor Discovery and ICMPv6

Neighbor Discovery Protocol uses ICMPv6 messages. Router Solicitation and Router Advertisement reveal routers and prefixes; Neighbor Solicitation and Neighbor Advertisement resolve neighbours and verify reachability. Duplicate Address Detection sends a Neighbor Solicitation with the unspecified source :: before a tentative address is used.

Interactive NDP and SLAAC journey

Follow address formation, DAD, router discovery, neighbour resolution, and delivery. Inspect each hop: ICMPv6 control messages build the information the host needs before normal traffic can move.

Watch IPv6 configure and find the next hop

Choose an IPv6 journey

Complete SLAAC journeyDAD conflictRouter discoveryNeighbour resolutionLocal or default router

Build an address, prove uniqueness, discover a router, resolve a neighbour, and deliver the packet.

• Host: Host eth0↔Switch: Switch Gi0/1
• Switch: Switch Gi0/2↔Router: Router Gi0/0
• Switch: Switch Gi0/3↔Peer: Peer eth0

Active: Host

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Step 1 of 8

1. Form a link-local address

The host forms a tentative fe80::/10 address before it has a globally scoped address.

Packet inspector

Inside the packet

Choose packet inspection depth

Plain-language viewTechnical inspection

Host or device decision

No frame is crossing a link during this step.

ICMPv6

Local address formation

IPv6 source

::

IPv6 destination

fe80::20c:29ff:fe9c:409

Technical packet details

Ethernet handling

Multicast is forwarded on eligible VLAN links; only group members accept it.

Local delivery or the default router

For an on-link destination, the host resolves the destination's link-layer address and sends directly. For an off-link destination, it resolves the default router's link-layer address; the IPv6 packet keeps the remote destination address while the first Ethernet frame targets the router.