IPv6: The Complete Guide
Everything IPv6 in one guide: the 128-bit address format → shortening rules → the five address types → SLAAC and NDP → subnetting → how IPv4 and IPv6 coexist. IPv6 is not "IPv4 with longer addresses" — several core behaviours change, and they are exactly what CCNA tests.
The address format and shortening rules
An IPv6 address is 128 bits: eight groups of four hex digits — 2001:0db8:0000:0000:0000:0000:0000:0001. Two rules shorten it: drop leading zeros in each group, and compress one run of all-zero groups to :: → 2001:db8::1. The :: can be used only once, or the address would be ambiguous.
The five address types
| Address type | Prefix | Purpose |
|---|---|---|
| Global unicast | 2000::/3 | Public, internet-routable |
| Link-local | fe80::/10 | Auto-configured, one link only — every interface has one |
| Unique local | fc00::/7 | Private (like RFC 1918) |
| Multicast | ff00::/8 | One-to-many (replaces broadcast) |
| Loopback | ::1/128 | The host itself |
The exam favourite: every interface gives itself a link-local (fe80::) address automatically — IPv6 routing protocols like OSPFv3 peer over link-locals, not global addresses.
No broadcast, no ARP: NDP
IPv6 removes broadcast entirely. The Neighbor Discovery Protocol (NDP), running over ICMPv6 multicast, replaces ARP (Neighbor Solicitation/Advertisement) and adds Router Solicitation/Advertisement — the machinery behind auto-configuration.
SLAAC vs DHCPv6
SLAAC (stateless auto-config): the router advertises the /64 prefix and hosts build their own address — no server, no lease table (try it in the SLAAC lab). DHCPv6 exists for when you need central control or extra options; commonly SLAAC assigns the address while DHCPv6 supplies DNS (the O-flag pattern).
Subnetting in IPv6
The LAN subnet is virtually always a /64. A site gets a /48 → 65,536 possible /64s; you subnet by walking hex digits on nibble boundaries. No usable-host arithmetic, no broadcast to subtract — full detail in IPv6 subnetting.
Coexisting with IPv4
The internet runs both stacks and will for years. Dual stack (device runs IPv4 + IPv6 side by side) is the standard approach; tunnelling and translation (NAT64) cover the edges. Compare protocols in IPv4 vs IPv6 — and note IPv6 needs no NAT: address abundance restores end-to-end connectivity, with firewalls (not address translation) providing the security boundary.
Frequently asked questions
How long is an IPv6 address?
128 bits, written as eight groups of four hexadecimal digits, shortened by dropping leading zeros and compressing one run of zero groups to ::.
What is a link-local address?
An fe80::/10 address every IPv6 interface auto-configures for communication on its own link. Routing protocols like OSPFv3 form adjacencies over link-local addresses.
Does IPv6 have broadcast?
No. IPv6 replaces broadcast with multicast, and the Neighbor Discovery Protocol (over ICMPv6) replaces ARP.
What is SLAAC?
Stateless Address Autoconfiguration — hosts build their own IPv6 address from the /64 prefix a router advertises, with no DHCP server required.
Why is there no NAT in IPv6?
Address abundance makes translation unnecessary — every device can have a globally unique address, with firewalls providing the security boundary NAT incidentally provided in IPv4.
Is IPv6 on the CCNA exam?
Yes — addressing, address types, SLAAC and IPv6 static routing are core CCNA 200-301 topics.
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