IPv6CCNA 200-301

IPv6 Wrong Prefix Length Causing Wrong Subnet Calculation

Presenting Symptom

Hosts on the same IPv6 subnet cannot communicate with each other, and pings to the default gateway fail.

Network Context

A small branch office with a Cisco 4321 router and a few PCs. The router interface GigabitEthernet0/0/0 is configured with IPv6 address 2001:db8:acad:1::1/64. PCs are configured with addresses in the 2001:db8:acad:1::/64 range but with a prefix length of /48 instead of /64, causing them to calculate a different subnet and fail to communicate.

Diagnostic Steps

Check IPv6 connectivity from a PC to the default gateway

ping 2001:db8:acad:1::1

Request timed out.

If ping fails, the PC may have incorrect IPv6 configuration or the router may not be reachable.

Verify IPv6 address and prefix length on the PC

ipconfig /all (Windows) or ifconfig (Linux/Mac)

IPv6 Address: 2001:db8:acad:1::100/48

The prefix length /48 is incorrect; it should be /64. This causes the PC to calculate a different subnet prefix (2001:db8:acad::/48) instead of the correct /64 subnet.

Check the router's IPv6 interface configuration

show ipv6 interface gigabitethernet 0/0/0

IPv6 is enabled, link-local address is FE80::1
Global unicast address(es):
  2001:db8:acad:1::1, subnet is 2001:db8:acad:1::/64

The router has the correct /64 prefix. The PC's /48 prefix does not match, so the PC considers the router to be on a different subnet.

Check the PC's routing table to see how it determines reachability

route print -6 (Windows) or ip -6 route (Linux)

2001:db8:acad::/48 via ::1 (loopback) or similar, no route to 2001:db8:acad:1::/64

The PC's routing table shows a route for its /48 prefix but not for the /64 subnet. It will not send traffic to the router because the router's address is not in the same /48 subnet.

Root Cause

The PC is configured with an IPv6 address that has a prefix length of /48 instead of /64. This causes the PC to calculate its subnet as 2001:db8:acad::/48, while the router's subnet is 2001:db8:acad:1::/64. Since the router's address is not within the PC's calculated subnet, the PC cannot send traffic to the router, resulting in no communication.

Resolution

Reconfigure the PC's IPv6 address with the correct prefix length of /64. On Windows: 1. Open Network and Sharing Center > Change adapter settings. 2. Right-click the network adapter > Properties. 3. Select Internet Protocol Version 6 (TCP/IPv6) > Properties. 4. Enter the IPv6 address: 2001:db8:acad:1::100 5. Enter the subnet prefix length: 64 6. Enter the default gateway: 2001:db8:acad:1::1 7. Click OK. On Linux (using netplan or ifconfig): - Edit /etc/netplan/01-netcfg.yaml: network: version: 2 ethernets: eth0: addresses: - 2001:db8:acad:1::100/64 gateway6: 2001:db8:acad:1::1 - Run sudo netplan apply On router (if misconfigured): - interface GigabitEthernet0/0/0 ipv6 address 2001:db8:acad:1::1/64

Verification

From the PC, ping the default gateway: ping 2001:db8:acad:1::1 Expected output: Reply from 2001:db8:acad:1::1: time<1ms Also verify the PC's IPv6 configuration: ipconfig /all (Windows) or ip addr show (Linux) Expected: IPv6 Address: 2001:db8:acad:1::100/64

Prevention

1. Use DHCPv6 or SLAAC to automatically assign correct prefix lengths to hosts. 2. Implement IPv6 prefix delegation to ensure consistent prefix lengths across the network. 3. Train staff on IPv6 addressing fundamentals, emphasizing the importance of the prefix length.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario tests understanding of IPv6 addressing and subnetting. Questions may present a troubleshooting scenario where hosts cannot communicate despite correct addresses, and the candidate must identify the prefix length mismatch. The exam may use drag-and-drop to match symptoms with causes or multiple-choice to select the correct prefix length.

Exam Tips

Always check the prefix length when troubleshooting IPv6 connectivity issues; a mismatch is a common mistake.

Remember that the prefix length defines the network portion; hosts must have the same prefix to communicate on the same link.

Know that show ipv6 interface displays the subnet prefix; compare it with the host's configuration.

Test Your CCNA Knowledge

Practice with scenario-based questions to prepare for the CCNA 200-301 exam.

Practice CCNA Questions