1: Which of the following are valid representations for the address 200A 0000 0000 0C00

0000 0000 0000 0000 with a 60-bit prefix?

A. 200A:0000:0000:0C/60

B. 200A::0C00:0:0:0:0/60

C. 200A:0000:0000:0C00::/60

D. 200A::0C00::/60

E. 200A:0:0:C00::/60

F. 200A::0C/60

A: B, C, E. A is not a complete address. D is ambiguous, with two sets of ::. F doesn't

expand to the correct address.

2: For what is the address 0:0:0:0:0:0:0:0 used?

A: This is the unspecified address. It represents the absence of an address. If this is the

source address of a packet, the interface has not yet been assigned an address. It is

attempting to discover whether its tentative address is being used by another node.

3: You configure your site border routers, connecting to an IPv6 public network, to

advertise all your internal network numbers, including FEC0:0020:0:0100::/56. You get

a nasty call from the IPv6 public network administrator. What is wrong?

A: FEC0:0020:0:0100::/56 is a site-local address. It must never be advertised beyond the

boundaries of a site.

4: Which extension headers are processed by every IPv6 node in the path from source to

destination?

A: Hop-by-Hop.

5: Which extension headers are used to specify a list of routers to visit before reaching the

destination and to have each of those routers process the header?

A: Destination Options header followed by the Routing header.

6: A router receives a packet larger than its outgoing link's MTU. Does it fragment the

packet and forward the fragments toward the destination?

A: No. It drops the packet and sends an ICMP Packet Too Big message back to the source.

The source uses these ICMP packets to perform path MTU discovery. It is the sole

responsibility of the source to fragment the packet.

7: If set in a Router Advertisement, what affect does the Managed bit have?

A: The router sends the RA to all hosts on a link. If the Managed bit is set, the hosts obtain

an address from a stateful configuration server.

8: If a router advertises prefix information in its RAs, how is the information used?

A: Prefix information included in RAs tells hosts which prefixes are on-link and/or which

prefixes to use when they autoconfigure their addresses.

9: In what two states can a host's IP address resides, and what are the roles of the two

states?

A: Preferred and deprecated. A preferred address can be used to initiate any IP session. A

deprecated address should be used only to maintain an existing connection, not to

initiate a new connection, if a preferred address exists.

10: What information does a router advertise in its RA to tell hosts to stop using a particular

prefix when initiating IP sessions?

A: Either a Valid Lifetime or Preferred Lifetime of value 0. Valid Lifetime 0 says the prefix is

no longer valid. Preferred Lifetime 0 says to deprecate the prefix.

11: If a node has a neighbor with state DELAY, can the node send the neighbor packets?

A: Yes. A neighbor with state DELAY has not been verified reachable, but the node will

send packets to its cached link layer address for the neighbor.

12: A host is not running any routing protocol. It is sending data to a remote node using a

default router. The default router fails. Will the host continue to send data into the black

hole of the dead router until its TCP connection fails?

A: No. The neighbor unreachability process, default router list, and address resolution

processes will assist the host in discovering the dead router and finding a new one.

13: What are the scope values for multicast packets, and for what are they used?

A: Node, link, site, organization, global. The scope values are used to limit the meaning of

a multicast group and to control how far a multicast packet can travel.

14: What Cisco router command enables IPv6 routing?

A: ipv6 unicast-routing

15: What interface subcommands enable IPv6 on an interface?

A: ipv6 enable, ipv6 address address prefix [eui-64]

16: What commands are used to enable a RIPng process?

A: Interface subcommand: ipv6 rip process-name enable

IPv6 Interview Q&A Cheat Sheet

1. What is IPv6 and why do we need it?

IPv6 is the next-generation Internet Protocol designed to replace IPv4.
We need it because IPv4 has only ~4.3 billion addresses, and they are almost exhausted. IPv6 provides 128-bit addresses, offering a vastly larger address space (~3.4×10³⁸ addresses).

2. How many bits are in an IPv6 address, and how is it represented?

IPv6 address = 128 bits (vs. 32 bits in IPv4).
Written as 8 groups of 4 hexadecimal digits, separated by colons. Example:
2001:0db8:85a3:0000:0000:8a2e:0370:7334.

3. What are the types of IPv6 addresses?

Unicast – one-to-one communication.
Multicast – one-to-many communication.
Anycast – one-to-nearest communication (delivered to the closest node).
No broadcast in IPv6 (replaced by multicast).

4. What is a Link-Local Address in IPv6?

Every IPv6-enabled device automatically gets a link-local address (fe80::/10).
It is valid only within the local link (network segment).
Required for communication between nodes on the same link (used by NDP).

5. What is the IPv6 loopback address?

IPv6 loopback: ::1
Equivalent of IPv4’s 127.0.0.1.
Used to test networking on the local machine.

6. What is SLAAC in IPv6?

Stateless Address Auto-Configuration (SLAAC) allows a host to automatically configure its own IPv6 address using Router Advertisements (RAs), without DHCP.

7. How does IPv6 handle ARP?

IPv6 replaces ARP with Neighbor Discovery Protocol (NDP), which uses ICMPv6 messages to find neighbors, routers, and resolve addresses.

8. What are Dual Stack and Tunneling in IPv6 migration?

Dual Stack: Devices run both IPv4 and IPv6 simultaneously.
Tunneling: Encapsulating IPv6 packets inside IPv4 to cross IPv4-only networks (e.g., 6to4, Teredo).

9. Does IPv6 support NAT?

NAT is not required in IPv6 due to the huge address space.
However, NAT64/DNS64 exists for IPv6–IPv4 interoperability.

10. What role does ICMPv6 play?

ICMPv6 is critical in IPv6.
Used for Neighbor Discovery (replaces ARP), error messages, Path MTU discovery, and Multicast Listener Discovery (MLD).

Quick Memory Hacks:

IPv6 = 128-bit, written in hex.
No broadcast → uses multicast.
Loopback = ::1.
Link-local = fe80::/10.
SLAAC = auto-configure with Router Advertisements.
ARP → replaced by NDP.

1. Why was IPv6 introduced when we already had IPv4?

The primary reason for creating IPv6 was IPv4 address exhaustion. IPv4 uses a 32-bit address, which allows for approximately 4.3 billion unique addresses. With the explosive growth of the internet and the proliferation of internet-connected devices (like smartphones, laptops, and IoT devices), the pool of available IPv4 addresses has been depleted. IPv6 uses a 128-bit address, providing a virtually limitless number of unique addresses (approximately 340 undecillion), thus solving the exhaustion problem for the foreseeable future.

2. What are the key differences between IPv4 and IPv6?

Feature	IPv4	IPv6
Address Size	32-bit	128-bit
Address Notation	Dotted-decimal (e.g., 192.168.1.1)	Hexadecimal, colon-separated (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334)
Address Configuration	Manual or via DHCP	Stateless Address Autoconfiguration (SLAAC) and DHCPv6
IPsec Support	Optional	Built-in and mandatory
Packet Header	More complex with variable length	Simpler, fixed-length for faster processing
Broadcast	Uses broadcast addresses	No broadcast; uses multicast and anycast instead
NAT	Heavily reliant on Network Address Translation (NAT)	NAT is not needed due to the vast address space

3. How do you represent an IPv6 address? Can it be shortened?

An IPv6 address is represented as eight groups of four hexadecimal digits, separated by colons.

For example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334

Yes, there are two main rules for shortening an IPv6 address:

Leading Zero Omission: You can omit leading zeros in any group.
- 0db8 can be written as db8.
- 00a0 can be written as a0.
Double Colon Rule: A single, contiguous block of all-zero groups can be replaced by a double colon (::). This can only be used once in an address.
- The address 2001:0db8:0000:0000:1234:0000:0000:5678 can be shortened to 2001:db8::1234:0:0:5678.
- The address fe80:0000:0000:0000:0202:b3ff:fe1e:8329 can be shortened to fe80::202:b3ff:fe1e:8329.

Applying both rules to the first example, 2001:0db8:85a3:0000:0000:8a2e:0370:7334 becomes 2001:db8:85a3::8a2e:370:7334.

4. What are the different types of IPv6 addresses?

There are three main types of IPv6 addresses:

Unicast: Identifies a single network interface. A packet sent to a unicast address is delivered to that specific interface.
- Global Unicast: Routable on the internet, similar to a public IPv4 address.
- Link-Local: Used for communication within a single network link. Automatically configured on all interfaces (starts with fe80::).
- Unique Local: Routable within a private network but not on the public internet, similar to private IPv4 addresses.
Multicast: Represents a group of interfaces (potentially on different devices). A packet sent to a multicast address is delivered to all interfaces in that group. This replaces the broadcast functionality of IPv4.
Anycast: An address assigned to a group of interfaces (usually on different devices). A packet sent to an anycast address is delivered to the nearest interface in the group, as determined by the routing protocol.

5. What is SLAAC (Stateless Address Autoconfiguration)?

SLAAC is a key feature of IPv6 that allows a device to automatically configure its own unique IPv6 address without the need for a DHCP server. The process works as follows:

The device first generates a link-local address for itself.
It then sends a Router Solicitation (RS) message onto the local network to find a router.
The router responds with a Router Advertisement (RA) message, which contains the network prefix.
The device combines the received network prefix with its own interface identifier (often derived from its MAC address using the EUI-64 process) to create a globally unique IPv6 address.

6. What are some IPv4 to IPv6 transition mechanisms?

Since the internet can't switch from IPv4 to IPv6 overnight, several mechanisms were developed to allow them to coexist and interoperate. The most common are:

Dual Stack: A device or network node runs both IPv4 and IPv6 protocol stacks simultaneously. It can communicate with both IPv4 and IPv6 networks directly. This is the most common and preferred method.
Tunneling: This involves encapsulating IPv6 packets within IPv4 packets to traverse an IPv4-only network. It's like putting an IPv6 letter into an IPv4 envelope to get it through the IPv4 postal system. Examples include 6to4, Teredo, and ISATAP.
Translation (NAT64): This mechanism allows IPv6-only devices to communicate with IPv4-only servers. A translator gateway removes the IPv6 header and replaces it with an IPv4 header (and vice versa), translating between the two protocols.

Simplifying Python, Networking, Security, and Virtualization concepts.

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Saturday, June 1, 2013

IPv6 Review Q&A