IPv6 uses 128-bit addresses and was designed to support a much larger address space along with updated packet and neighbor-discovery behavior. IPv4 and IPv6 are separate protocols; dual-stack hosts can run both while networks transition.
Subnetting · Lesson 7
IPv6
Learn how to read IPv6 addresses, prefixes, scopes, autoconfiguration, and Linux routing state.
Reading IPv6 Notation
An IPv6 address is written as eight 16-bit hexadecimal groups:
2001:0db8:0000:0000:0000:0000:0000:0025
Leading zeros in each group can be omitted, and one consecutive run of zero groups can be compressed with :::
2001:db8::25
Only one :: may appear because otherwise the number of omitted groups would be ambiguous. 2001:db8::/32 is reserved for documentation examples.
Why can :: appear at most once in an IPv6 address?
Address Types and Scope
Important addresses and ranges include:
::1/128: loopback on the local host.fe80::/10: link-local unicast; normally present on IPv6 interfaces.2000::/3: currently allocated global unicast space.ff00::/8: multicast.
IPv6 has no broadcast address; multicast and Neighbor Discovery serve use cases that IPv4 often handles with broadcast. A link-local destination can require an interface zone such as fe80::1%eth0 because the same prefix exists on every link.
What is the normal scope of an fe80::/10 address?
Prefixes and Interface Addresses
IPv6 CIDR notation uses a prefix length from /0 through /128. A /64 is the standard size for most LAN subnets and supports Stateless Address Autoconfiguration. An interface can hold link-local, stable global, temporary privacy, and other addresses simultaneously, each with preferred and valid lifetimes.
Why might one interface show several IPv6 addresses?
Neighbor and Router Discovery
IPv6 Neighbor Discovery uses ICMPv6 for address resolution, duplicate-address detection, router discovery, and reachability information. Router Advertisements can provide prefixes and default-router information. Hosts may combine SLAAC with DHCPv6 for other configuration; DHCPv6 normally does not supply the default router.
Blocking all ICMPv6 breaks essential protocol behavior. Firewall policy should permit the required message types with appropriate scope rather than treating ICMPv6 as optional.
How does an IPv6 host normally learn a default router dynamically?
Inspecting and Testing IPv6
Inspect addresses, routes, and neighbors independently:
$ ip -6 address show
$ ip -6 route show
$ ip -6 neighbor show
$ ping -6 -c 3 2001:db8::25
Use a real assigned test address rather than the documentation address shown. A dual-stack application can succeed over IPv4 while IPv6 is broken, or the reverse, so test each family and its DNS A or AAAA records explicitly.
Why test IPv4 and IPv6 separately on a dual-stack service?
Lesson complete
You finished IPv6
You can now read and test common IPv6 interface and routing state.
Expand or compress eight hexadecimal address groups correctly.
Distinguish loopback, link-local, global, and multicast scope.
Expect several IPv6 addresses and lifetimes on one interface.
Preserve required Neighbor Discovery and Router Advertisement traffic.
Test IPv4 and IPv6 paths independently on dual-stack services.
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