IPv6 Addressing and Neighbor Discovery Quick Reference for CCNA 200-301 v2.0

IPv6 troubleshooting becomes easier when you separate four questions:

1. What kind of address is this?
2. Which prefix identifies the local network?
3. How was the interface identifier created?
4. Can the device discover its neighbor and a usable path?

Use a short IPv6 decision order

Start with live state rather than rebuilding the address from memory.

1. Confirm the interface is up.
2. Confirm the expected global or unique-local address and prefix are present.
3. Confirm a link-local address exists.
4. Check the default gateway or route.
5. Check Neighbor Discovery state.
6. Test the local link before testing a remote destination.
7. Check the return path.

Useful starting commands are:

show ipv6 interface brief
show ipv6 interface GigabitEthernet0/0/0
show ipv6 neighbors
show ipv6 route

Match the address type to its job

Every IPv6-enabled interface normally has a link-local address. That address supports local neighbor communication and can be used as a routing next hop when the outgoing interface is also clear.

Read prefixes and compress addresses carefully

A LAN commonly uses a /64 prefix. The first 64 bits identify the subnet, and the last 64 bits identify the interface within that subnet.

Do not apply IPv4 host-count habits to ordinary IPv6 LANs. IPv6 subnet design usually emphasizes a clear address hierarchy rather than saving individual addresses.

To shorten an IPv6 address:

• Remove leading zeros inside a hexadecimal group.
• Replace one continuous run of all-zero groups with ::.
• Use :: only once.

2001:0db8:0010:0020:0000:0000:0000:0042
2001:db8:10:20::42

To expand the shortened address, restore eight groups and four hexadecimal digits per group.

Recognize modified EUI-64

Modified Extended Unique Identifier 64-bit (EUI-64) can build a 64-bit interface identifier from a 48-bit MAC address.

For MAC address 00:1A:2B:3C:4D:5E:

1. Split the MAC into two halves.
2. Insert FF:FE between them.
3. Flip the universal/local bit in the first byte. 00 becomes 02.
4. Combine the result with the /64 prefix.

00:1A:2B:FF:FE:3C:4D:5E
021A:2BFF:FE3C:4D5E

With prefix 2001:db8:10:20::/64, the full address becomes:

2001:db8:10:20:21a:2bff:fe3c:4d5e

Cisco IOS XE can create the interface identifier with:

interface GigabitEthernet0/0/0
 ipv6 address 2001:db8:10:20::/64 eui-64

Modern endpoints may use privacy-oriented or randomly generated interface identifiers. The exam still expects you to recognize the modified EUI-64 process.

Neighbor Discovery replaces several IPv4 local-link functions

Neighbor Discovery Protocol (NDP) uses Internet Control Message Protocol version 6 (ICMPv6). It helps devices find local neighbors, learn routers, resolve Layer 3 addresses to Layer 2 addresses, and detect certain reachability problems.

If ICMPv6 is blocked too broadly, IPv6 can fail in ways that look unrelated because Neighbor Discovery depends on it.

Separate SLAAC, DHCPv6, and manual configuration

Stateless Address Autoconfiguration (SLAAC) lets a host build an address from a prefix learned in a Router Advertisement.

DHCPv6 can provide addresses or other configuration depending on the design and Router Advertisement flags.

Manual configuration assigns the address directly on the device.

A client can have a valid link-local address but no usable global address. That usually means the local IPv6 stack is running, while prefix advertisement, address assignment, or policy still needs attention.

Treat link-local next hops as local-link addresses

A router can use a global address or a link-local address as an IPv6 next hop. When a link-local next hop is used, the outgoing interface must also be specified because the same link-local range exists on every IPv6 link.

ipv6 route 2001:db8:30::/64 GigabitEthernet0/0/1 FE80::2

A default route matches destinations that do not have a more-specific route:

ipv6 route ::/0 2001:db8:12::2

If the router must forward IPv6 traffic, enable routing globally:

ipv6 unicast-routing

Choose IOS evidence that answers one question

Scenario comparisons

The host has FE80:: but no global address

The local IPv6 stack is active. Check Router Advertisements, SLAAC or DHCPv6 design, VLAN placement, and filtering before changing the link-local address.

A static route uses FE80::2 and does not install

Confirm the outgoing interface is included and is operational. A link-local next hop needs interface context.

Two addresses look different but represent the same value

Expand both to eight groups before comparing them. Compression changes the notation, not the address.

Local-link ping works but remote traffic fails

Check IPv6 routing, the default route, forwarding state, and the return path. Local neighbor success does not prove end-to-end routing.

Common exam traps

• Treating a link-local address as globally routable.
• Expecting IPv6 broadcast instead of multicast.
• Using :: more than once in one address.
• Applying IPv4 host-count optimization to ordinary IPv6 /64 LANs.
• Forgetting the outgoing interface with a link-local static-route next hop.
• Assuming a link-local address proves that global address assignment succeeded.
• Blocking ICMPv6 without considering Neighbor Discovery.

Rapid review grid

Official references

• Cisco CCNA 200-301 v2.0 exam topics
• Cisco: Understand the IPv6 Link-Local Address
• IETF RFC 4291: IP Version 6 Addressing Architecture
• IETF RFC 4861: Neighbor Discovery for IP version 6