Networking Concepts · Topic 10 of 11
IPv6 Addressing and Transition
Learn IPv6 address format and shortening rules, address types, neighbor discovery, and the dual stack, tunneling and NAT64 methods used with IPv4.
- 5 min
- An IPv6 address is 128 bits, written in hexadecimal in eight groups.
- Leading zeros can be dropped, and one run of zero groups can become a double colon only once.
- Link local addresses begin with fe80 and exist on every IPv6 interface.
What it is
Internet Protocol version 6 (IPv6) uses 128 bit addresses, which solves the shortage of IPv4 addresses. An IPv6 address is written as eight groups of four hexadecimal digits separated by colons.
IPv6 also changes how hosts find neighbors and routers. It removes broadcast and adds new address types, and the exam expects you to recognize the formats and the ways IPv6 coexists with IPv4.
How it works
Address format and shortening
A full address looks like 2001:0db8:0000:0000:0000:0000:0000:0001. Two rules shorten it.
Leading zeros in a group can be dropped. One run of consecutive all zero groups can be replaced with a double colon, and this can be done only once per address.
Full: 2001:0db8:0000:0000:0000:0000:0000:0001
Shortened: 2001:db8::1The first 64 bits are normally the network prefix and the last 64 bits are the interface identifier. A /64 is the standard size for a subnet.
One way to build the interface identifier is EUI-64 (Extended Unique Identifier, 64 bit). The host splits its 48 bit MAC address in the middle, inserts the hex value FFFE, and flips the seventh bit of the first byte.
Address types
| Type | Prefix or value | Purpose |
|---|---|---|
| Global unicast | 2000::/3 | Public, routable on the internet |
| Unique local | fc00::/7 (in practice fd00::/8) | Private use inside an organization, like IPv4 private space |
| Link local | fe80::/10 | Valid on one link only, created automatically on every interface |
| Multicast | ff00::/8 | Delivery to a group |
| Anycast | Any unicast address shared by several hosts | Delivery to the nearest one |
| Loopback | ::1 | The local host |
| Unspecified | :: | A host with no address yet |
Every IPv6 interface has a link local address, and it is used for neighbor discovery and for routers to exchange routing information. A device often has several addresses at once, such as a link local and a global one.
Neighbor discovery and address assignment
The Neighbor Discovery Protocol (NDP) replaces the Address Resolution Protocol (ARP) of IPv4. It uses Internet Control Message Protocol version 6 (ICMPv6) messages to find the hardware (Media Access Control, MAC) addresses of neighbors and to find routers.
With Stateless Address Autoconfiguration (SLAAC), a host listens for a router advertisement, learns the prefix and builds its own address without a server. Stateful assignment uses the Dynamic Host Configuration Protocol (DHCP) for IPv6 (DHCPv6), covered with network services.
Mitigating address exhaustion
IPv4 addresses ran out because of the 32 bit space. IPv6 offers a vastly larger space, so every device can have a global address and the workaround of address translation becomes unnecessary.
Compatibility with IPv4
IPv6 and IPv4 hosts cannot talk directly, so three techniques help during the transition.
| Method | How it works |
|---|---|
| Dual stack | A device runs IPv4 and IPv6 at the same time and uses the one the destination supports |
| Tunneling | IPv6 packets are carried inside IPv4 packets across an IPv4 only network |
| NAT64 | A translator (Network Address Translation from IPv6 to IPv4) lets IPv6 clients reach IPv4 only servers, usually with DNS64 (a Domain Name System feature) to supply synthesized addresses |
Dual stack is the simplest and most common approach, but it means managing two sets of addresses, policies and firewall rules.
What to remember for the exam
- An IPv6 address is 128 bits, written in hexadecimal in eight groups.
- Leading zeros can be dropped, and one run of zero groups can become a double colon only once.
- Link local addresses begin with fe80 and exist on every IPv6 interface.
- Global unicast starts with 2000::/3, unique local with fc00::/7, and multicast with ff00::/8.
::1 is loopback.
is the unspecified address.
- IPv6 has no broadcast. NDP replaces ARP.
- SLAAC lets a host build its own address from a router advertisement.
- Dual stack, tunneling and NAT64 are the transition methods.
- A /64 is the standard subnet size for a local network. EUI-64 builds an interface identifier from the MAC address by inserting FFFE.
To expand a shortened address, count the groups that are present, then insert enough zero groups at the double colon to reach eight.
Common mistakes
Using two double colons. A double colon may appear only once, or the address would be ambiguous.
Expecting broadcast. IPv6 uses multicast and anycast. An all nodes multicast address replaces what a broadcast did.
Mixing up link local and unique local. Link local (fe80) is limited to one link and not routed. Unique local (fc00::/7) can be routed inside a site but not on the internet.
Thinking translation is needed for scale. IPv6 removes address shortage, and security should come from firewalls rather than from Network Address Translation (NAT).
Assuming dual stack means translation. Dual stack runs both protocols natively, with no conversion between them.
A device that only has a link local address can talk on its own link but cannot reach other networks. Check for a global address and a default router when IPv6 connectivity is missing.
Check yourself
Answer these three.
How do you shorten 2001:0db8:0000:0000:0000:0000:0000:0001?
Drop the leading zeros and replace the longest run of zero groups with a double colon, giving 2001:db8::1.
Which protocol replaces ARP in IPv6?
The Neighbor Discovery Protocol (NDP), which uses ICMPv6.
Which transition method runs IPv4 and IPv6 side by side on the same device?
Dual stack.
Topic 10 of 11 in Networking Concepts
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