Section 30: IPv6 Addressing and Routing

What are the rules for writing IPv6 addresses in shortened form?

Leading zeros in each group can be omitted, and one contiguous run of all-zero groups can be replaced with :: (only once per address).

What range are Global Unicast Addresses assigned from?

2000::/3

What is dual stack, and how is it used in IPv4-to-IPv6 transition?

A network interface running both IPv4 and IPv6 addresses simultaneously — used as a transition strategy allowing gradual migration.

Does IPv6 support broadcast traffic? What replaces it?

No — IPv6 has no broadcast. Multicast to all hosts on the local subnet serves the same functional purpose.

What subnet mask does IPv6 standard practice recommend for individual host subnets?

/64

How many bits is an IPv6 address vs. IPv4?

IPv6 = 128 bits, IPv4 = 32 bits.

What range are IPv6 Link-Local addresses assigned from?

FE80::/10 – FEB0::/10

What is the IPv6 equivalent of ARP?

Neighbor Discovery Protocol (NDP).

What mask do local IPv6 routes always use?

/128

What message types does Neighbor Discovery use, and what ICMP version are they part of?

Neighbor Solicitation and Neighbor Advertisement messages, part of ICMPv6.

Are Link-Local addresses mandatory on IPv6-enabled interfaces?

Yes.

What are Unique Local Addresses, and what IPv4 concept are they similar to?

Similar to RFC 1918 private addresses — not publicly routable/reachable.

What is the scope of a Link-Local address?

Valid for communication on that local link only — not routable beyond it.

What command enables IPv6 routing?

ipv6 unicast-routing

What is the IPv6 equivalent of the IPv4 default route (0.0.0.0/0)?

::/0

What does the IPv6 address ::/128 mean, and when is it used?

"Unspecified"/"unknown" address — used as a source address when an interface is in the process of acquiring an address.

What Neighbor Discovery message is sent to the Solicited-Node multicast address?

Neighbor Solicitation

Why does standardizing on /64 everywhere simplify IPv6 addressing?

It enables consistent use of EUI-64 for automatic host address generation.

How does EUI-64 generate the host portion of an IPv6 address from a MAC address?

FF:FE is inserted into the middle of the 48-bit MAC address to extend it to 64 bits, and the 7th bit of the MAC address is inverted.

With SLAAC, how does a host request addressing information from the router?

By sending a Router Solicitation message.

What is a key limitation of NAT that IPv6 solves, especially relevant to multimedia applications like VoIP?

NAT breaks the end-to-end IP model — applications that embed IP addresses/port numbers in higher-layer protocol data (like VoIP signaling) can fail because the embedded private address isn't reachable outside the local network, and NAT doesn't rewrite those embedded values.

Why can :: only be used once in a shortened IPv6 address?

Using it more than once creates ambiguity about how many zero groups belong on each side — the address couldn't be reliably expanded back to its original form.

If a company is assigned a /48 block and uses /64 for every subnet, how many subnets can they create?

65,536 (16 bits available between the /48 and /64 boundaries)

Why is it not recommended to use EUI-64 to generate IPv6 addresses on router interfaces?

It produces long, unmemorable addresses derived from the MAC address, making troubleshooting harder — manually defined addresses are preferred on routers for clarity.

What happens if EUI-64 is configured on a non-Ethernet interface (like Serial) that has no MAC address of its own?

The router borrows the MAC address from its first Ethernet port to generate the address.

What range are Unique Local addresses assigned from?

FC00::/7

What happens in IPv4 if you enter a second ip address command on an interface without the secondary keyword?

It overwrites the first address — only the most recently entered one takes effect. The secondary keyword is required to have two IPv4 addresses simultaneously (max two).

How does IPv6 differ from IPv4 in how many addresses can be assigned to one interface?

IPv6 allows multiple addresses per interface with no special keyword needed (unlike IPv4's two-address max requiring secondary) — typically one Link-Local plus one Global Unicast is standard on routers.

What critical piece of information can SLAAC NOT provide to a host, and why does this matter?

The DNS server address — SLAAC's original design has no mechanism for it, so a DHCPv6 server is still typically needed even when SLAAC handles IP addressing.

Why do modern operating systems avoid using EUI-64 (MAC-derived) addresses when self-generating an IPv6 address via SLAAC?

Privacy — a MAC-derived address lets a device be tracked across networks, so modern OSes generate a randomized host portion instead.

Are IPv4 and IPv6 routing handled by the same routing table and process on a router?

No — they're completely separate processes with separate routing tables, even on a dual-stack router.

If ipv6 unicast-routing is NOT enabled, can a router still communicate with directly connected IPv6 hosts?

Yes — it can respond to hosts on its own connected subnets, but it will NOT forward/route IPv6 traffic between different subnets, which can be confusing since local connectivity still works.