IPv4 has about 4.3 billion addresses, which are exhausted. IPv6 uses 128-bit addresses written as eight groups of four hex digits: 2001:0db8:0000:0000:0000:ff00:0042:8329.
Shortening rules: drop leading zeros in a group; replace one run of all-zero groups with :: (only once). The address above becomes 2001:db8::ff00:42:8329.
Differences that matter for beginners
- Every IPv6 interface gets a link-local address (
fe80::/10) automatically. It works only on that link and is used for Neighbour Discovery and router messages even before a global address exists. - There is no broadcast. One-to-many uses multicast; the jobs ARP did in IPv4 are handled by Neighbour Discovery (ND) with ICMPv6.
- A LAN is usually numbered with a /64 prefix. SLAAC lets hosts build their own address from the router advertisement; DHCPv6 can still be used when administrators want tighter control.
- During the long migration, many networks run dual-stack (IPv4 and IPv6 together). Tunnelling and translation are fallback transition tools when one side is IPv6-only.
Tip: recognise fe80:: as link-local and ::1 as loopback at a glance. If a host only has link-local addresses, it cannot reach the Internet yet — it still needs a global or unique-local address and a route.
Common mistake: using :: more than once in one address. Only a single run of zero groups may be compressed, or the address is ambiguous.
Remember
- IPv6 = 128 bits; shorten with dropped leading zeros and one
::. - Link-local always; no broadcast; ND replaces ARP.
- /64 is the normal LAN size; dual-stack is the usual transition path.
| Type | Prefix | Note |
|---|---|---|
| Global unicast | 2000::/3 | Public, routable |
| Link-local | fe80::/10 | Automatically on every interface, local link only |
| Unique local | fc00::/7 | Private-style addressing |
| Multicast | ff00::/8 | Replaces broadcast |
| Loopback | ::1 | Local host |
IPv6 has no broadcast and usually uses a /64 prefix for a LAN. Addresses can be assigned by SLAAC (auto-configuration) or DHCPv6. Transition methods: dual-stack, tunnelling, translation.
Check yourself. When you have finished this unit, try the 4.7 hands-on lab and the 4.8 review questions.