IPv6 Fundamentals

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

Where the address sits in an IPv6 packet

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.

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.

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.

Inspect IPv6 evidence

On Windows, use ipconfig /all, Get-NetIPAddress -AddressFamily IPv6, Get-NetRoute -AddressFamily IPv6, and Get-NetNeighbor -AddressFamily IPv6. On Linux, use ip -6 address, ip -6 route, and ip -6 neighbour. In Wireshark, start broadly with ipv6, then isolate Neighbor Discovery with icmpv6 or message filters such as icmpv6.type == 135.

Practise IPv6 interpretation

For each supplied address, expand all eight hextets, apply canonical shortening, classify its type and scope, split the prefix at the stated length, and derive the solicited-node multicast address. Then decide whether the first hop targets the neighbour or the default router.

Troubleshoot IPv6 methodically

1. Confirm the interface has a usable link-local address and is not stuck tentative after DAD.
2. Verify the prefix length and on-link route instead of judging by similar-looking text.
3. Check Router Advertisements for prefix, lifetime, and default-router information.
4. Inspect the neighbour cache and NS/NA exchange for the selected next hop.
5. Check ICMPv6 filtering; blocking essential control messages can break IPv6 even when addresses look correct.

Knowledge check and summary

Pro deep dive