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Flashcards in IPv6 Deck (101):

What is the size in bits of the IPv4 address scheme?

IPv4 is based on a 32-bit address scheme.


Which classes of IPv4 addresses are not considered globally unique unicast IP addresses?

Class D (multicast) and Class E (experimental) are not used as globally unique unicast IP addresses by hosts and routers on the Internet. Although the private addresses are within the range of unicast addresses, they must not be considered globally unique, because many organizations use them inside their network only.


What is the main rationale behind IPv6?

The IPv4 address scheme is limited by being 32-bit.

Some parts of the address scheme cannot be used as globally unique unicast addresses (Class D, Class E, loopback,, private addresses).

Very large blocks of addresses are assigned to organizations.

The size of the global Internet routing table is huge.

NAT is deployed everywhere. It saves globally unique unicast addresses but breaks IP's end-to-end model.

The Internet is still growing.

Address exhaustion is projected to occur during the current decade.

There is enough time to design an update of the current protocol.


Explain the consequences of the exhaustion of the IPv4 address space.

New IPv4 address space will be impossible to get.

There are not enough globally unique unicast addresses for every device, but the number of new devices requiring IP addresses (such as PDAs and cell phones) is increasing.


Name some limitations of NAT.

It breaks the end-to-end model of IP.

The network must handle connections and state.

It causes problems for networks with fast rerouting, links, and route redundancy.

It hinders network performance.

Keeping records of all connections becomes mandatory for providers and organizations that must have records for security reasons.

NAT modifies the IP header. This affects end-to-end security protocols such as IPSec AH.

Applications that are not NAT-friendly cannot pass through NAT.

Address space collisions are frequent when organizations merge networks. Private addressing is recommended behind NAT.


Describe some of the features added by IPv6.

Plenty of IP addresses are available for the next decades.

Multiple levels of hierarchy provide efficient and scalable routing to the Internet.

Multihoming with route aggregation.

Autoconfiguration allows nodes to configure their IPv6 addresses.

A renumbering mechanism provides transparency when customers change IPv6 providers.

ARP broadcast is replaced by multicast.

The IPv6 header is more efficient than IPv4

Fewer fields

A Flow Label field for traffic differentiation

New extension headers replace IPv4's Options field

Mobility and security mechanisms are built into IPv6.

Transition mechanisms help networks move from IPv4 to IPv6.


What is the size in bits of an IPv6 address?

An IPv6 address has 128 bits. This is 4 times more bits than in IPv4 addresses.


Comparing the OSI reference model of IPv4 to that of IPv6, which layer is updated?

IPv6 has a change at Layer 3 (the network layer). Upper and lower layers are lightly modified to handle IPv6.


With plenty of IP addresses with IPv6, what is not desirable to have?

NAT is undesirable in IPv6.


Define aggregation.

Aggregation is a synonym for route summarization, which is a consolidation of routes in a routing table. The main benefit of aggregation is the reduction of routes in a routing table.


What happens when a customer changes IPv6 providers?

IPv6 space allocated to a customer is part of the ISP's IPv6 space. To keep a strict aggregation in IPv6, which is desirable for the Internet global routing table, the customer must change its IPv6 prefixes each time it changes providers.


Why is multihoming more interesting with IPv6 than with IPv4?

Multihoming is possible in both IPv4 and IPv6. But in IPv6, multihoming is made possible by keeping strict route aggregation in the Internet global routing table.


Explain autoconfiguration.

A router on the local link sends network information to all nodes. Nodes listen to this information and can configure their own IPv6 addresses.


Besides autoconfiguration, name the other methods used to configure IPv6 addresses on nodes.

Static configuration (manually), DHCPv6, using random interface identifiers


Describe the disadvantages of ARP broadcast in IPv4.

ARP broadcast requests cause many interrupts in every node connected on a local link. An ARP broadcast is sent to the IP stack through the interface and the operating system.


List the main change in the IPv6 header compared to IPv4.

The IPv4 packet length is 20 bytes compared to 40 bytes for IPv6.

Fewer fields are present in the IPv6 header.

The IPv4 Header Checksum field is removed.

Fragmentation is handled differently in IPv6, so fields related to the fragmentation are either gone or are replaced by extension headers.

The Flow Label field is added for traffic differentiation.

The IPv4 header Options field is replaced by several extension headers.


What is the purpose of an extension header?

It provides better efficiency in the options processing, because each extension header ensures that routers and nodes compute only headers targeted for them.


List and define two mechanisms that are embedded in the IPv6 protocol but that are considered add-ons with IPv4.

Mobile IP lets nodes move from one IP network to another while keeping the same IP addresses.

IPSec enables end-to-end security over IP networks.


How is the migration from IPv4 to IPv6 different from the Y2K bug?

The Y2K bug was a major shift scheduled for a specific date. The migration from IPv4 to IPv6 will be a smooth transition over several years.


List the fields removed from the IPv4 header that are not in IPv6

Header length, Identification, Flags, Fragment Offset, Header checksum, Options, Padding


What new field is added in the IPv6 header?

Flow Label


Describe the use of the Next Header field in the IPv6 header.

The Next Header field defines the type of information following the basic IPv6 header. The type of information can be a transport-layer protocol such as TCP or UDP, or it can be an extension header.


List the extension headers that may be placed after the basic IPv6 header, and place them in the order they must appear.

IPv6 header

Hop-by-Hop Options header

Destination Options header (if the Routing header is used)

Routing header

Fragment header

Authentication header

Encapsulating Security Payload header

Destination Options header

Upper-layer header (TCP, UDP, ICMPv6, ...)


What is mandatory with UDP when used over IPv6?

The UDP Checksum field within the UDP packet is mandatory with IPv6. This field was optional in IPv4.


What is recommended as a mechanism for nodes in IPv6 to avoid fragmentation?

The path MTU discovery (PMTUD) mechanism


What are IPv6's minimum MTU and recommended minimum MTU?

The minimum MTU in IPv6 is 1280 octets, and the recommended minimum MTU is 1500 octets.


What are the three representations of IPv6 addresses?

Preferred representation (always a series of eight 16-bit hexadecimal fields)

Compressed representation (successive 16-bit fields made up of 0s are replaced by a double colon; leading 0s in 16-bit fields are removed)

IPv6 address with an embedded IPv4 address (used by transition mechanisms)


Describe the IPv6 address representation for URL.

Because the colon specifies an optional port number in a URL, the IPv6 address must be enclosed in brackets.


List the three kinds of addresses in the IPv6 addressing architecture.

Multicast, unicast, anycast


IPv6 address: Unspecified



IPv6 address: Loopback



IPv6 address: IPv4 Compatible IPv6



IPv6 address: Link Local



IPv6 address: Site Local



IPv6 address: Multicast



IPv6 address: Solicited node multicast



IPv6 address: Aggregatable global unicast



What is a link-local address?

Link-local addresses are used by nodes on a local link scope only. These addresses cannot be routed between segments. Each IPv6 node by default owns one link-local address per network interface.


What is similar to the site-local address in IPv4?

Site-local addresses are similar to the private addressing space in IPv4, such as,, and Site-local addresses must not be routed to the IPv6 Internet.


Give the length in bits of the host and site parts of an aggregatable global unicast IPv6 address.




What three prefixes are assigned by IANA and are available as public addresses in IPv6?

2001::/16—IPv6 Internet

2002::/16—6to4 transition mechanism



What is the Cisco IOS Software command to enable IPv6 on a Cisco router?

ipv6 unicast-routing


What protocol ID is used for IPv6 in Ethernet frames?



Explain how IPv6 multicast addresses are mapped over Ethernet.

Multicast mapping over Ethernet uses the multicast Ethernet prefix to which the low-order 32-bit of the IPv6 addresses is appended.


What command assigns one IPv6 address to an interface using EUI-64 format?

ipv6 address ipv6-address/prefix-length eui-64


What is the goal of the path MTU discovery mechanism?

The main goal of the path MTU discovery mechanism is to find out the maximum MTU value along a path when a packet is sent.


What is the goal of stateless autoconfiguration?

Stateless autoconfiguration allows nodes on a local link to assign their IPv6 addresses by themselves. Network information is advertised by a router on the local link.


List the main information carried by the router advertisement message when a prefix is advertised.

IPv6 prefix

Valid and preferred lifetimes

Default router information



What command displays the prefix advertisement parameters on an interface?

show ipv6 interface interface prefix


What command overrides the default prefix advertisement parameters on an interface?

ipv6 nd prefix


What is duplicate address detection (DAD)?

Before assigning an IPv6 address to an interface, each node must verify that the address it wants to use is unique and is not already in use by another node.
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What new DNS record was added for IPv6?



What are the implicit rules in an extended IPv6 ACL?

permit icmp any any nd-ns

permit icmp any any nd-na

deny ipv6 any any


What commands and tools are available in IOS IPv6 to diagnose problems and manage a router?

Commands available are ping and traceroute. Tools are Telnet, SSH, TFTP, and HTTP. All of them include IPv6 support.


What command displays the whole IPv6 routing table?

The command show ipv6 route displays the current IPv6 routes of the IPv6 routing table.


What changes were made to BGP4+ for IPv6 support?

The NEXT_HOP attribute can be expressed as an IPv6 address. Also, this attribute can contain both a global and link-local IPv6 address.

The NLRI can be expressed as an IPv6 prefix.


What command is used in the BGP router subcommand mode to disable the advertisement of routing information for the IPv4 address family?

The no bgp default ipv4-unicast command disables the advertisement of IPv4 information by BGP4+.


How should an IPv6 prefix list be applied in a BGP4+ configuration?

An IPv6 prefix list is applied to a BGP neighbor in the address-family ipv6 router subcommand mode.


What destination address is used by RIPng in IPv6 to send updates?

The destination address is the multicast address FF02::9. This multicast address is an all-rip-routers multicast address on the link-local scope.


Which command enables RIPng on an interface?

ipv6 rip tag enable


Which command enables an OSPFv3 process on the router?

ipv6 router ospf


What replaces the network area command in OSPFv3?

This command is replaced by a new way to identify an IPv6 network. The command ipv6 ospf process-id area area-id is now used on an interface basis to perform this task.


List the three classes of integration and coexistence strategies presented in this chapter.

Dual stack, tunneling, protocol translation mechanism


Describe the dual-stack approach.

Dual stack is a way for nodes on a network to handle and use the IPv4 and IPv6 protocols simultaneously.


What type of Ethernet frame is made by IPv6-only applications on nodes?

The Ethernet frame used is 0x86DD in the protocol-ID field instead of 0x0800 for IPv4.


How do IPv4- and IPv6-enabled applications choose the IP stack when both IPv6 and IPv4 stacks are available?

The end user can enter either the IPv4 or IPv6 address.

The node can use the naming service (DNS) to select the stack.


Which type of address is preferred by IPv4- and IPv6-enabled applications when the naming service provides both IPv4 (A-record) and IPv6 (AAAA-record) address types?

The IPv6 address is preferred by IPv4 and IPv6-enabled applications.


When should you consider using integration and coexistence mechanisms?

Integration and coexistence mechanisms should be considered as an alternative only when getting native IPv6 connectivity on networks and links is not possible.


What is the protocol number defined for the encapsulation of IPv6 packets in IPv4?



List the three scenarios in which the tunneling of IPv6 packets in IPv4 is possible.

Host to host, host to router, router to router


What is the main requirement of tunneling?

Tunneling is possible on nodes that have dual-stack support.


List all the tunneling techniques presented in this chapter.

Configured tunnel, tunnel broker (based on configured tunnel), tunnel server (based on configured tunnel), 6to4, GRE tunnel, ISATAP, automatic IPv4-compatible tunnel


What is the main characteristic of a configured tunnel?

It is configured manually on the dual-stack node.


What is the purpose of the tunnel broker and the tunnel server?

Tunnel broker and tunnel server are mechanisms that automate the deployment and configuration of configured tunnels.


Describe how the prefix to a 6to4 site is assigned.

The prefix of a 6to4 site is made by appending the IPv4 address of the 6to4 router to prefix 2002::/16. The IPv4 address of the 6to4 router is converted to hexadecimal. The final representation is 2002:ipv4-address::/48.


What is the purpose of the 6to4 relay?

Basically, 6to4 sites can route traffic within the 2002::/16 prefix only. The 6to4 relay is a gateway between the IPv4 Internet and the IPv6 Internet. Therefore, 6to4 sites on the IPv4 Internet can exchange IPv6 traffic with the IPv6 Internet through a 6to4 relay.


Define the ISATAP address format.

The ISATAP address format is created from the concatenation of a prefix and the interface ID in ISATAP format. The link-local prefix of an ISATAP address is FE80::/10. One aggregatable global unicast prefix must be dedicated to the ISATAP operation within a domain (all ISATAP devices must use the same prefix). The interface ID (low-order 64-bit) in the ISATAP format is made by appending the IPv4 address of the ISATAP host or ISATAP router to the hexadecimal value 0000:5EFE. The final representation of an ISATAP address is prefix:0000:5EFE:ipv4-address.


Describe how the ISATAP unicast prefix is advertised to the ISATAP host by an ISATAP router.

The ISATAP host starts by sending a router solicitation to an ISATAP router through an ISATAP tunnel (over IPv4). After receiving the router solicitation, the ISATAP router sends a router advertisement containing the ISATAP unicast prefix to the ISATAP host through an ISATAP tunnel (over IPv4). Finally, the ISATAP host uses the aggregatable global unicast prefix received to autoconfigure its unicast IPv6 address.


Does the IPv4-compatible tunnel mechanism provide a solution to the IPv4 address space exhaustion?

Because the IPv4-compatible tunnel mechanism is based on the global unicast IPv4 addresses, this mechanism does not offer a solution to the IPv4 address space exhaustion.


List the two methods that allow IPv6-only nodes in an IPv6-only network to communicate with IPv4-only nodes in an IPv4-only network.

Application-Level Gateway (ALG), NAT-PT


List the different types of operations defined for the NAT-PT mechanism.



What is the purpose of the 96-bit prefix for the NAT-PT mechanism?

A /96 prefix must be reserved in an IPv6 domain for the NAT-PT operation. All IPv6 packets addressed to the /96 prefix must be routed to the NAT-PT device. Then the NAT-PT device translates IPv6 addresses into IPv4 addresses according to its mapping rules


Define anycast

An IPv6 address type that is used by a number of hosts in a network that are providing the same service. Hosts accessing the service are routed to the nearest host in an anycast environment based on routing protocol metrics.


Define multicast

A type of IPv4 and IPv6 traffic designed primarily to provide one-to-many connectivity but unlike broadcast, has the capability to control the scope of traffic distribution.


Define MLD

The IPv6 protocol used for the discovery of which hosts are listening for which multicast IP addresses for IPv6.


Define stateless autoconfiguration

A method used by an IPv6 host to determine its own IP address, without DHCPv6, by using NDP and the modified EUI-64 address format.


Define link-local

An address type in IPv6 networks that is used only on the local link and never beyond that scope


Define stateful autoconfiguration

A method of obtaining an IPv6 address that uses DHCPv6.


Define EUI-64

A specification for the 64-bit interface ID in an IPv6 address, composed of the first half of a MAC address, hex FFFE, and the last half of the MAC.


Define ND

The protocol used in IPv6 for many functions, including address autoconfiguration, duplicate address detection, router, neighbor, and prefix discovery, neighbor address resolution, and parameter discovery.


Define RA

In IPv6, a Router Advertisement message used by an IPv6 router to send information about itself to nodes and other routers connected to that router.


Define NA

In IPv6, the Neighbor Discovery message used by an IPv6 node to send information about itself to its neighbors.


Define NS

In IPv6, the Neighbor Discovery message used by an IPv6 node to request information about a neighbor or neighbors.


Define solicited-node multicast

In IPv6, an address used in the Neighbor Discovery (ND) process. The format for these addresses is FF02::1:FF00:0000/104, and each IPv6 host must join the corresponding group for each of its unicast and anycast addresses.


Define AAAA

In IPv6 DNS, the IPv6 equivalent of an IPv4 DNS A record.


Define SSM

Receivers subscribe to an (S,G) channel when they request to join a multicast group. That is, they specify the unicast IP address of their multicast source and the group multicast address. SSM is typically used in very large multicast deployments such as television video.


Define BSR

A standards-based way of helping routers find Rendezvous Points (RP). RPs notify BSRs of the groups they handle. BSRs in turn flood the group-to-RP mappings throughout the network. Each router individually determines which RP to use for a particular group.


Define 6to4

An IPv6/IPv4 tunneling method that allows isolated IPv6 domains to be connected over an IPv4 network.



An IPv6/IPv4 tunneling method that is designed for transporting IPv6 packets within a site where a native IPv6 infrastructures is not available.


Define NAT-PT

a method of translating between IPv4 and IPv6 that removes the need for hosts to run dual protocol stacks. NAT-PT is an alternative to tunneling IPv6 over an IPv4 network, or vice versa.


Define GRE

A tunneling protocol that can be used to encapsulate many different protocol types, including IPv4, IPv6, IPsec, and others, to transport them across a network.