CCNA IPv6 Practice Questions: Addressing, EUI-64, SLAAC and NDP

How to work these questions

1. To compress: write all eight groups, drop the leading zeros in each group (0db8 becomes db8, 0000 becomes 0), then find the longest run of consecutive 0 groups and replace it with ::. If two runs tie, compress the first one; never compress a lone 0 group when a longer run exists, and never use :: twice.
2. To expand: count the groups written on each side of the ::, subtract from eight, and that is how many 0000 groups the :: stands for. Pad each group back to four digits. You should end with exactly eight groups; if not, you miscounted.
3. For EUI-64, split the MAC into two halves of 24 bits and insert fffe in the middle: 5c26.0a7b.9e31 becomes 5c26:0aff:fe7b:9e31.
4. Then write the first byte in binary and invert the seventh bit from the left. 5c is 0101 1100, so it becomes 0101 1110, which is 5e. A quick check: the flip always changes the second hex digit by 2 (0 to 2, 2 to 0, 4 to 6, c to e, d to f, and so on).
5. Prepend the /64 prefix and compress the result; the router shows it in uppercase with leading zeros removed, so 0aff appears as AFF.
6. For subnet counting, subtract the prefix lengths: a /48 split into /64s has 16 subnet bits, so 2^16 = 65,536 subnets, and the subnet ID is simply the fourth group of the address.

Practice questions

Pick an answer to see right away whether it is correct and why every option is right or wrong.

1. Question 1 of 10Calculation

   Write 2001:0db8:0000:0000:0000:00a1:0000:0042 in its shortest valid form.

   1. 2001:db8::a1:42
   2. 2001:db8::a1:0:42
   3. 2001:db8::a1::42
   4. 2001:db8::a1:0:0:42
   Show the answer

   Answer: B

   Compression has two rules: drop leading zeros inside each 16-bit group, and replace one run of consecutive all-zero groups with ::. When there are several runs, :: goes on the longest one; a single zero group is written as 0. To check your work, expand the result back and count eight groups.

   • A. Expanding this gives 2001:db8:0:0:0:0:a1:42, which is a different address: the single zero group between a1 and 42 has been lost.
   • B. Leading zeros drop from every group (0db8 becomes db8, 00a1 becomes a1, 0042 becomes 42), and only the longest run of all-zero groups, the three in the middle, collapses to ::. The single zero group before 42 stays written as 0.
   • C. Only one :: is allowed in an address; with two, there is no way to tell how many zero groups each one stands for.
   • D. This expands to 2001:db8:0:0:a1:0:0:42, with two zero groups before 42 instead of one; here the :: stands for only two groups, so a1 lands in the wrong position.

2. Question 2 of 10Calculation

   Interface G0/0 has the MAC address 5c26.0a7b.9e31 and is configured with ipv6 address 2001:db8:10:20::/64 eui-64. Which global unicast address does the interface get?

   1. 2001:db8:10:20:5c26:aff:fe7b:9e31
   2. 2001:db8:10:20:5d26:aff:fe7b:9e31
   3. 2001:db8:10:20:5e26:a7b:fffe:9e31
   4. 2001:db8:10:20:5e26:aff:fe7b:9e31
   Show the answer

   Answer: D

   EUI-64 builds a 64-bit interface ID from a 48-bit MAC: split the MAC in half, insert fffe in the middle, and invert the seventh bit of the first byte. Write the first byte in binary before flipping; 0x00 becomes 0x02, 0x5c becomes 0x5e, 0x02 becomes 0x00. The router then prepends the configured /64 prefix.

   • A. This keeps the first byte as 5c, which skips the mandatory flip of the seventh bit; fffe is in the right place, but the interface ID is wrong.
   • B. 5c is 0101 1100; changing the last bit gives 5d, but EUI-64 flips the seventh bit from the left (the universal/local bit), not the eighth.
   • C. fffe must go between the third and fourth bytes of the MAC, between the OUI half and the device half, not after the fourth byte.
   • D. Split 5c26.0a7b.9e31 into 5c260a and 7b9e31, insert fffe to get 5c26:0aff:fe7b:9e31, then flip the seventh bit of 5c (0101 1100 becomes 0101 1110, which is 5e). Dropping the leading zero in 0aff gives 5e26:aff:fe7b:9e31.

3. Question 3 of 10Calculation

   A server has the address 2001:db8:5a00:10c7:21e:4fff:fe9a:1b2c/64, carved from the company's 2001:db8:5a00::/48 allocation. Which /64 is the server on, and how many /64 subnets does the /48 contain in total?

   1. 2001:db8:5a00:10c7::/64; 65,536 subnets
   2. 2001:db8:5a00:10c7::/64; 4,096 subnets
   3. 2001:db8:5a00::/64; 65,536 subnets
   4. 2001:db8:5a00:10c7:21e::/64; 256 subnets
   Show the answer

   Answer: A

   In IPv6 the prefix length tells you where the subnet ends: with /64, the first four groups are the network and the last four are the interface ID. A /48 site allocation leaves 16 bits for subnetting, so it holds 65,536 /64 networks, from 2001:db8:5a00:0::/64 to 2001:db8:5a00:ffff::/64. SLAAC needs exactly /64, which is why this layout is the standard one.

   • A. The first four groups (64 bits) are the prefix, so the subnet is 2001:db8:5a00:10c7::/64. Between /48 and /64 there are 16 subnet bits, and 2^16 = 65,536.
   • B. The prefix is right, but 4,096 is 2^12; the subnet field is the whole fourth group, which is 16 bits, not 12.
   • C. 2001:db8:5a00::/64 is only the first subnet of the block (subnet ID 0000). The server's fourth group is 10c7, so it sits in a different /64.
   • D. A /64 boundary falls after the fourth 16-bit group; 21e belongs to the interface ID, not the prefix. 256 (2^8) would need an 8-bit subnet field.

4. Question 4 of 10Scenario

   While checking a host's IPv6 configuration you see the following four addresses associated with its network interface. Which one is a global unicast address?

   1. fe80::5a26:aff:fe7b:9e31
   2. fd12:3456:789a:1::25
   3. 2001:db8:5a00:10c7::25
   4. ff02::1:ff00:25
   Show the answer

   Answer: C

   Read the first hex digits: 2 or 3 is global unicast (2000::/3), fc or fd is unique local (fc00::/7), fe8 to feb is link-local (fe80::/10), ff is multicast (ff00::/8), and ::1 is loopback. Every interface has a link-local address regardless of what else is configured, and a packet to the internet needs a global unicast source. Anycast addresses look like ordinary unicast addresses; they are only special because more than one device is configured with them.

   • A. fe80::/10 is link-local: every IPv6 interface creates one, routers never forward it, and it is only valid on the local segment.
   • B. fd12:... is inside fc00::/7, the unique local range, the IPv6 counterpart of private addressing; it is not routed on the internet.
   • C. The first hex digit is 2, which places the address in 2000::/3, the global unicast range. 2001:db8::/32 is the slice of it reserved for documentation, which is why it appears in exam questions.
   • D. ff02::1:ff00:25 is the solicited-node multicast group the host joined for its addresses ending in 00:0025; it is a group it listens to, not a unicast address of its own.

5. Question 5 of 10Exhibit

   Hosts on the LAN behind G0/0 need both an IPv6 address and a DNS server address. Based on the exhibit, how do they obtain them?

   R1# show ipv6 interface GigabitEthernet0/0
   GigabitEthernet0/0 is up, line protocol is up
     IPv6 is enabled, link-local address is FE80::2D0:1AFF:FE2B:3C4D
     No Virtual link-local address(es):
     Global unicast address(es):
       2001:DB8:1:10::1, subnet is 2001:DB8:1:10::/64
     Joined group address(es):
       FF02::1
       FF02::2
       FF02::1:FF00:1
       FF02::1:FF2B:3C4D
     MTU is 1500 bytes
     ICMP error messages limited to one every 100 milliseconds
     ICMP redirects are enabled
     ICMP unreachables are sent
     ND DAD is enabled, number of DAD attempts: 1
     ND reachable time is 30000 milliseconds (using 30000)
     ND advertised reachable time is 0 (unspecified)
     ND advertised retransmit interval is 0 (unspecified)
     ND router advertisements are sent every 200 seconds
     ND router advertisements live for 1800 seconds
     ND advertised default router preference is Medium
     Hosts use stateless autoconfig for addresses.
     Hosts use DHCP to obtain other configuration.

   1. Address and DNS server both from a stateful DHCPv6 server
   2. Address from SLAAC; DNS server from the prefix option in the RA
   3. Address from stateful DHCPv6; DNS server from the RA
   4. Address from SLAAC; DNS server from a stateless DHCPv6 server
   Show the answer

   Answer: D

   The last lines of show ipv6 interface tell you what the router advertises: the M flag (ipv6 nd managed-config-flag) sends hosts to stateful DHCPv6 for addresses, the O flag (ipv6 nd other-config-flag) sends them to stateless DHCPv6 for options only, and with neither set hosts rely on SLAAC alone. SLAAC gives an address but, on its own, no DNS server unless the RA carries the RDNSS option. Stateless DHCPv6 fills that gap without the server tracking any addresses.

   • A. Stateful DHCPv6 would need the M flag, which IOS reports as hosts using DHCP to obtain routable addresses. The exhibit says addresses come from stateless autoconfig.
   • B. The prefix information option carries the /64 that hosts use to build their own address; it does not carry DNS servers. The other-configuration line points to DHCPv6 for the rest.
   • C. This is backwards: the RA here provides the prefix for SLAAC, and the last line says hosts turn to DHCP for the other configuration, not the other way round.
   • D. The last two lines show the O flag set and the M flag clear: hosts build their address with SLAAC from the advertised prefix and then ask a DHCPv6 server only for options such as DNS, which is stateless DHCPv6.

6. Question 6 of 10Scenario

   PC1 (2001:db8:1:10::50/64) sends its first packet to the default gateway 2001:db8:1:10::1, whose MAC address it does not yet know. What does PC1 send to learn it?

   1. A Neighbor Solicitation to ff02::1:ff00:1
   2. A Router Solicitation to ff02::2
   3. An ARP request to the all-nodes address ff02::1
   4. A Neighbor Advertisement to 2001:db8:1:10::1
   Show the answer

   Answer: A

   NDP replaces ARP with ICMPv6 messages: Neighbor Solicitation and Neighbor Advertisement for address resolution and duplicate address detection, Router Solicitation and Router Advertisement for discovering routers and prefixes. The NS goes to the solicited-node multicast address, which every node computes from its own addresses, so only a handful of hosts ever see it. The NA comes back unicast and fills the neighbor cache, shown with show ipv6 neighbors.

   • A. IPv6 resolves MAC addresses with an ICMPv6 Neighbor Solicitation sent to the solicited-node multicast group: ff02::1:ff00:0/104 plus the last 24 bits of the target, here 00:0001. Only hosts whose addresses end in those 24 bits process it.
   • B. A Router Solicitation asks routers to send a Router Advertisement right away, so the host can learn the prefix and default router; it does not resolve a specific neighbor's MAC address.
   • C. ARP does not exist in IPv6 and neither does broadcast; ff02::1 is the all-nodes multicast group, but address resolution uses the far narrower solicited-node group.
   • D. A Neighbor Advertisement is the reply: the gateway sends it back to PC1 with its MAC address in the target link-layer address option.

7. Question 7 of 10Scenario

   R1 has ipv6 unicast-routing enabled and G0/0 is configured with 2001:db8:1:10::1/64. Which two multicast groups does G0/0 join as a result of that configuration alone? (Choose two.)

   1. ff02::5
   2. ff02::1
   3. ff02::1:2
   4. ff02::2
   5. ff02::9
   Show the answer

   Answer: B and D

   ff02 means link-local scope, so these groups never leave the segment. All nodes join ff02::1 and the solicited-node group for each of their unicast addresses; routers additionally join ff02::2. Routing protocols add their own groups (ff02::5 and ff02::6 for OSPFv3, ff02::a for EIGRP, ff02::9 for RIPng). You can check the list under Joined group address(es) in show ipv6 interface.

   • A. ff02::5 is the all-OSPF-routers group; an interface joins it only once OSPFv3 is enabled on it.
   • B. Every IPv6 node joins the all-nodes group ff02::1 on each interface; this is where Router Advertisements are sent.
   • C. ff02::1:2 is All_DHCP_Relay_Agents_and_Servers; a router joins it only when it acts as a DHCPv6 server or relay.
   • D. A device that forwards IPv6, which is what ipv6 unicast-routing turns on, joins the all-routers group ff02::2 and listens there for Router Solicitations.
   • E. ff02::9 is the RIPng group, joined only when RIPng is configured on the interface.

8. Question 8 of 10Config

   R1 connects to R2 through GigabitEthernet0/1. R2's interface on that link has the link-local address fe80::2, and R1 must reach 2001:db8:2::/64 through R2. Which command installs a working static route?

   1. ipv6 route 2001:db8:2::/64 fe80::2 name R2
   2. ip route 2001:db8:2::/64 GigabitEthernet0/1 fe80::2
   3. ipv6 route 2001:db8:2::/64 GigabitEthernet0/1 fe80::2
   4. ipv6 route 2001:db8:2::/64 fe80::2 GigabitEthernet0/1
   Show the answer

   Answer: C

   A fully specified IPv6 static route names the exit interface and the next hop: ipv6 route prefix/length interface next-hop. When the next hop is a global address the interface is optional, because the router can resolve it recursively. When the next hop is link-local it is mandatory, since fe80::/10 addresses are only unique per link. Using the neighbor's link-local address is common practice because it does not change if the global addressing of the link changes.

   • A. fe80::2 may exist on every link, so IOS cannot tell which interface to use; it rejects the command with a message that an interface has to be specified for a link-local next hop, and the name tag changes nothing.
   • B. ip route configures the IPv4 routing table and does not accept an IPv6 prefix. The IPv6 equivalent is ipv6 route.
   • C. With a link-local next hop the exit interface is mandatory, and IOS takes it before the address: prefix, exit interface, next hop. R1 sends the packet out G0/1 and resolves fe80::2 with NDP on that link.
   • D. IOS expects the exit interface before the next-hop address; with the order reversed, the parser reads fe80::2 as the next hop on its own and does not accept the interface that follows.

9. Question 9 of 10Exhibit

   PC1 on the LAN behind R1 G0/0 gets only a link-local address, even though it is set to autoconfigure IPv6. Based on the exhibit, what is the cause?

   R1# show running-config | section interface GigabitEthernet0/0
   interface GigabitEthernet0/0
    no ip address
    ipv6 address 2001:DB8:1:10::1/64
   R1# show running-config | include unicast-routing
   R1# show ipv6 interface brief
   GigabitEthernet0/0     [up/up]
       FE80::2D0:1AFF:FE2B:3C4D
       2001:DB8:1:10::1
   GigabitEthernet0/1     [up/up]
       FE80::2D0:1AFF:FE2B:3C4E
       2001:DB8:1:12::1

   1. The interface is missing the ipv6 enable command
   2. ipv6 unicast-routing is missing, so R1 sends no RAs
   3. A /64 is too long for SLAAC; hosts need a shorter prefix
   4. PC1 builds a random interface ID instead of using EUI-64
   Show the answer

   Answer: B

   ipv6 unicast-routing is the switch that turns a Cisco router into an IPv6 router. Without it the device can have IPv6 addresses and reach things itself, but it does not forward IPv6 packets and does not send Router Advertisements, so SLAAC on the connected LANs silently stops working. When hosts have only fe80 addresses, check this command before anything else. show ipv6 interface brief confirms the addresses are up but says nothing about forwarding.

   • A. ipv6 enable is only needed to bring up IPv6 on an interface with no address; configuring a global address already enables IPv6 and creates the link-local address, and the brief output confirms both are present.
   • B. The include filter returns nothing, so ipv6 unicast-routing is not configured. Without it IOS behaves as a host: it does not send Router Advertisements, so PC1 never learns the prefix or a default gateway and keeps only its self-generated link-local address.
   • C. SLAAC requires exactly a 64-bit prefix, because the host appends a 64-bit interface ID; /64 is the right length, not a problem.
   • D. Whether the host uses EUI-64 or a random interface ID only changes the low 64 bits; it still needs a prefix from an RA to build a global address at all.

10. Question 10 of 10Exhibit

    R1 receives a packet addressed to 2001:db8:2:7::9. Based on the routing table in the exhibit, what does R1 do with it?

    R1# show ipv6 route
    IPv6 Routing Table - default - 9 entries
    Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
           B - BGP, R - RIP, H - NHRP, I1 - ISIS L1
           I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary, D - EIGRP
           EX - EIGRP external, ND - ND Default, NDp - ND Prefix, DCE - Destination
           NDr - Redirect, RL - RPL, O - OSPF Intra, OI - OSPF Inter
           OE1 - OSPF ext 1, OE2 - OSPF ext 2, ON1 - OSPF NSSA ext 1
           ON2 - OSPF NSSA ext 2, la - LISP alt, lr - LISP site-registrations
           ld - LISP dyn-eid, lA - LISP away, a - Application
    S   ::/0 [1/0]
         via FE80::3, GigabitEthernet0/2
    C   2001:DB8:1:10::/64 [0/0]
         via GigabitEthernet0/0, directly connected
    L   2001:DB8:1:10::1/128 [0/0]
         via GigabitEthernet0/0, receive
    C   2001:DB8:1:12::/64 [0/0]
         via GigabitEthernet0/1, directly connected
    L   2001:DB8:1:12::1/128 [0/0]
         via GigabitEthernet0/1, receive
    C   2001:DB8:1:13::/64 [0/0]
         via GigabitEthernet0/2, directly connected
    L   2001:DB8:1:13::1/128 [0/0]
         via GigabitEthernet0/2, receive
    S   2001:DB8:2::/48 [1/0]
         via 2001:DB8:1:12::2
    L   FF00::/8 [0/0]
         via Null0, receive

    1. Forwards it out G0/2 to fe80::3 using the default route
    2. Forwards it out G0/0 because 2001:db8:2:7::/64 is directly connected
    3. Drops it because there is no /64 route for 2001:db8:2:7::
    4. Forwards it out G0/1 to 2001:db8:1:12::2 using the /48 static route
    Show the answer

    Answer: D

    show ipv6 route follows the same longest-match rule as IPv4: the route with the most matching prefix bits wins, and ::/0 is the last resort. C entries are connected networks, L entries are the router's own addresses as /128 host routes, and S entries are static. A static route with a global next hop and no interface is resolved recursively against a connected route; the FF00::/8 entry via Null0 is normal and just means multicast is handled locally.

    • A. The default route matches everything, but 2001:db8:2::/48 also matches and is more specific; longest prefix wins, so the default is only used when nothing else matches.
    • B. The connected networks are 2001:db8:1:10::/64, 2001:db8:1:12::/64 and 2001:db8:1:13::/64; 2001:db8:2:7::9 is in none of them.
    • C. A route does not need to be a /64 to be used: 2001:db8:2::/48 covers every address whose first 48 bits are 2001:0db8:0002, including 2001:db8:2:7::9.
    • D. The first 48 bits of 2001:db8:2:7::9 match 2001:db8:2::/48, the longest matching prefix. The next hop 2001:db8:1:12::2 is inside the connected 2001:db8:1:12::/64 on G0/1, so the router resolves it there and forwards out G0/1.

Common mistakes

• Using :: twice, or using it on the shorter of two zero runs; the address either becomes ambiguous or changes meaning.
• Flipping the wrong bit in EUI-64: it is the seventh bit from the left of the first byte, which changes the second hex digit by 2, not the first digit and not the last bit.
• Writing an IPv6 static route with a link-local next hop and no exit interface; IOS rejects it because fe80::/10 addresses are not unique beyond the link.
• Forgetting ipv6 unicast-routing and then hunting for an interface problem when hosts only have fe80 addresses; the router is simply not sending Router Advertisements.
• Treating ff02::1 as a broadcast and expecting ARP; IPv6 has no broadcast, and address resolution is a Neighbor Solicitation to the solicited-node multicast group.

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