200-301 Question #1533: Real Exam Question with Answer & Explanation

Question

Lab Simulation 30 Guidelines This is a lab item in which tasks will be performed on virtual devices. - Refer to the Tasks tab to view the tasks for this lab item. - Refer to the Topology lab to access the device console(s) and perform the tasks. - Console access is available for all required devices by clicking the device icon or using the tab(s) above the console window. - All necessary preconfigurations have been applied. - Do not change the enable password or hostname for any device. - Save your configurations to NVRAM before moving to the next item. - Click Next at the bottom of the screen to submit this lab and move to the next question. - When Next is clicked, the lab closes and cannot be reopened. Topology Configure IPv4 and IPv6 between the two routers. Tasks Reference Topology Diagram and table. Configure IPv4 and IPv6 between the two routers. Task 1: - Configure R1 with the first usable host IP address in the IPv4 network. - Configure R2 with the last usable host IP address in the IPv4 network. - Verify connectivity using ping. Task 2: - Do not assign the subnet router anycast address to either router. - Configure R1 with the first usable host IP address in the IPv6 network. - Configure R2 with the last usable host IP address in the IPv6 network. - Verify connectivity using ping. Answer: Task 1: Configure IPv4 between R1 and R2 From the table, the IPv4 subnet is 192.168.168.192/28. This gives us 16 total IP addresses, with 14 usable host IPs in the range 192.168.168.193 to 192.168.168.206. The first usable address is 192.168.168.193, and the last usable address is 192.168.168.206. R1 Configuration (First Usable IPv4 Address): R1(config)# interface Ethernet0/1 R1(config-if)# ip address 192.168.168.193 255.255.255.240 R1(config-if)# no shutdown R2 Configuration (Last Usable IPv4 Address): R2(config)# interface Ethernet0/1 R2(config-if)# ip address 192.168.168.206 255.255.255.240 R2(config-if)# no shutdown Verify IPv4 Connectivity: From R1, ping R2's IP address to ensure connectivity: R1# ping 192.168.168.206 Task 2: Configure IPv6 between R1 and R2 From the table, the IPv6 subnet is 2001:db8:12::/125. This provides 8 addresses, with the range 2001:db8:12::1 to 2001:db8:12::6 as usable addresses. The first usable address is 2001:db8:12::1, and the last usable address is 2001:db8:12::6. R1 Configuration (First Usable IPv6 Address): R1(config)# interface Ethernet0/1 R1(config-if)# ipv6 address 2001:db8:12::1/125 R1(config-if)# no shutdown R2 Configuration (Last Usable IPv6 Address): R2(config)# interface Ethernet0/1 R2(config-if)# ipv6 address 2001:db8:12::6/125 R2(config-if)# no shutdown Verify IPv6 Connectivity: From R1, ping R2's IPv6 address to ensure connectivity: R1# ping ipv6 2001:db8:12::6 IPv4 Addressing: The subnet 192.168.168.192/28 provides 16 IP addresses, with the first usable being 192.168.168.193 (assigned to R1) and the last usable being 192.168.168.206 (assigned to R2). IPv6 Addressing: The subnet 2001:db8:12::/125 provides 8 addresses, and the first usable is 2001:db8:12::1 (assigned to R1) and the last usable is 2001:db8:12::6 (assigned to R2). Both IPv4 and IPv6 configurations are done without using the subnet router anycast address.

Options

• taskConfigure IPv4 and IPv6 between the two routers (R1 and R2) based on the provided topology and IP scheme.
• prerequisitesAccess to virtual device consoles (R1, R2). All necessary preconfigurations have been applied. Do not change the enable password or hostname for any device.

Explanation

Lab Simulation 30: Configuring IPv4 and IPv6 Between Two Routers

Overall Goal

The objective is to establish Layer 3 connectivity between two routers by assigning IP addresses from specific subnets to each router's interface. Both IPv4 and IPv6 must be configured. This is a foundational networking task - without correct IP addressing, no routing, pinging, or higher-level communication is possible between these devices.

Conceptual Foundation Before the Steps

IPv4 Subnetting - 192.168.168.192/28:

• /28 = subnet mask 255.255.255.240 (last octet: 11110000)
• Block size: 16 addresses
• Network address: 192.168.168.192 (all host bits = 0) - not assignable
• Broadcast address: 192.168.168.207 (all host bits = 1) - not assignable
• Usable host range: .193 → .206 (14 hosts)
• R1 gets .193 (first usable), R2 gets .206 (last usable)

IPv6 Subnetting - 2001:db8:12::/125:

• /125 = 3 host bits → 2³ = 8 addresses
• Network/subnet-router anycast address: 2001:db8:12::0 - must not be assigned (task explicitly forbids it)
• "Broadcast" equivalent in IPv6 (all-ones host): 2001:db8:12::7 - not usable (used internally by IPv6 for subnet-router anycast in some interpretations, but regardless it's the last address)
• Usable range: ::1 → ::6 (6 addresses)
• R1 gets ::1 (first usable), R2 gets ::6 (last usable)

Note on IPv6 anycast: In IPv6, there is no broadcast. Instead, the all-zeros host address (::0 in this subnet) is the subnet-router anycast address, which routers use for certain routing functions. Assigning it to an interface would cause ambiguous behavior. That's why the task explicitly says "do not assign the subnet router anycast address."

Step-by-Step Explanation

Step 1 - Identify IPv4 Parameters

You must do the subnet math before touching the router. Miscalculating the usable range means assigning the network address (.192) or broadcast (.207) to an interface, which Cisco IOS will reject outright, or assigning an address outside the subnet entirely.

The /28 mask is the critical input. Memorize: /28 = 16 addresses, 14 usable. The first usable is always network address + 1, the last usable is always broadcast - 1.

Step 2 - Configure R1 for IPv4

R1(config)# interface Ethernet0/1
R1(config-if)# ip address 192.168.168.193 255.255.255.240
R1(config-if)# no shutdown

• interface Ethernet0/1: Enters interface configuration mode for the correct physical port. If you configure the wrong interface, the address goes nowhere useful.
• ip address ...: Assigns the IP. You must specify both the address and the subnet mask - IOS needs the mask to know the network boundary.
• no shutdown: Cisco interfaces default to administratively down. Skipping this means the interface stays disabled and pings will fail even with a correct address assigned. This is one of the most common mistakes beginners make.

Step 3 - Configure R2 for IPv4

R2(config)# interface Ethernet0/1
R2(config-if)# ip address 192.168.168.206 255.255.255.240
R2(config-if)# no shutdown

Same reasoning as Step 2. Both routers must be in the same subnet for direct communication - that's why both use the /28 mask. If R2 were given a different mask (e.g., /24), it would compute a different network boundary and might not recognize R1 as a neighbor on the same segment.

Step 4 - Verify IPv4 Connectivity

R1# ping 192.168.168.206

This confirms the physical link is up, both interfaces are up/up, and addresses are correctly assigned and reachable. If the ping fails:

• Check show interface Ethernet0/1 - is it up/up?
• Check show ip interface brief - is the address correct?
• Common culprits: forgot no shutdown, wrong interface, typo in address

Verifying before moving to IPv6 isolates problems to one protocol at a time.

Step 5 - Identify IPv6 Parameters

/125 is uncommon and easy to miscalculate. The key insight:

• /125 leaves 3 bits for the host portion → 2³ = 8 addresses
• In hex, the last group ::0 through ::7 are the 8 addresses
• ::0 = subnet-router anycast (forbidden by the task)
• ::7 = last address (equivalent concept to broadcast in IPv4 context)
• ::1 through ::6 = 6 usable addresses

Step 6 - Configure R1 for IPv6

R1(config)# interface Ethernet0/1
R1(config-if)# ipv6 address 2001:db8:12::1/125
R1(config-if)# no shutdown

• ipv6 address (not ip address): IPv6 is a separate protocol stack. You can have both an IPv4 and IPv6 address on the same interface simultaneously (dual-stack) - they don't conflict.
• The prefix length /125 is written inline with the address (unlike IPv4 which uses a separate mask argument).
• no shutdown is still required for the same reason as IPv4.

If you were to accidentally assign 2001:db8:12::0/125 (the anycast address), some IOS versions will allow it but it creates routing ambiguity - packets meant for the subnet itself could be misdirected.

Step 7 - Configure R2 for IPv6

R2(config)# interface Ethernet0/1
R2(config-if)# ipv6 address 2001:db8:12::6/125
R2(config-if)# no shutdown

Same reasoning. Note ::6 is the last usable address - ::7 is avoided because it's the last address in the /125 block (analogous to broadcast in function).

Step 8 - Verify IPv6 Connectivity

R1# ping ipv6 2001:db8:12::6

You must specify ipv6 in the ping command, otherwise IOS defaults to IPv4 and will fail (or try to resolve it as an IPv4 address). This is a frequent exam mistake.

If the IPv6 ping fails but IPv4 works, check:

• show ipv6 interface brief - is IPv6 enabled and address correct?
• Is ipv6 unicast-routing needed? (In some topologies, this must be enabled globally for IPv6 routing to work, though for a direct link ping it may not be required.)

Step 9 - Save to NVRAM

R1# copy running-config startup-config
R2# copy running-config startup-config

(or write memory / wr)

The running configuration lives in RAM. If the router reboots without saving, all your work is lost. In a lab exam, failing to save means your configuration may not be evaluated correctly. Never skip this step.

What Goes Wrong If Steps Are Skipped

Memory Tips

IPv4 usable range formula:

First usable = Network + 1 | Last usable = Broadcast - 1 | Broadcast = Network + (2^host_bits - 1)

IPv6 /125 shortcut:

3 host bits → 8 addresses → usable ::1 to ::6 (skip ::0 anycast, skip ::7 last)

Dual-stack interfaces:

ip address = IPv4 | ipv6 address = IPv6 | Both can coexist on one interface

The "no shut" rule:

Every time you configure a Cisco interface, end with no shutdown. Always. No exceptions in a lab.

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