Cisco Certified Network Associate (CCNA) 200-301 · Free study guide

Objective 1.8 — Configure IPv6 addressing and prefixes

IPv6 uses 128-bit addresses written as eight hexadecimal groups. Accurate work depends on preserving group positions, applying compression rules correctly, and treating the prefix as a routing boundary rather than as decoration.

Expand and compress safely

Each hexadecimal group represents 16 bits and can contain four digits. Leading zeroes inside a group may be removed. One contiguous run of all-zero groups may be replaced by ::, and :: may appear only once in an address because the remaining group count must be unambiguous.

To expand an address, count explicit groups and insert enough zero groups at :: to reach eight. To compare two compressed addresses, expand them first. For example, 2001:db8:4::25 expands to 2001:0db8:0004:0000:0000:0000:0000:0025.

Prefix meaning

A prefix length identifies the leading network bits. /64 is common on IPv6 LANs, leaving a 64-bit interface identifier, but never assume a missing prefix. Two addresses share an on-link prefix only if the declared leading bits match. There is no IPv6 broadcast address; neighbor discovery and other group delivery use multicast.

Interface assignment

An IOS-style interface can receive a manually configured IPv6 address and prefix. IPv6 unicast routing must be enabled when the device is expected to route between interfaces. Link-local addresses also exist on IPv6-enabled interfaces and are important for local-link protocol operation.

Choose unique addresses from the plan, apply the prefix, enable the interface, and avoid reusing an interface identifier accidentally. Address configuration does not create remote routes beyond connected prefixes.

Verification

Inspect the interface's global and link-local addresses, prefix, and line state. Confirm connected and local IPv6 routes. Test the directly connected neighbor before a remote destination. When using a link-local next hop, the exit interface may be required because the same link-local range appears on every link.

Client tools on modern operating systems can display multiple temporary and stable addresses. Identify which address and source-selection behavior are relevant before declaring a duplicate or wrong address.

IOS command map

On a lab router, enter interface GigabitEthernet0/1, configure ipv6 address 2001:db8:120:7::1/64, and use no shutdown. Enable ipv6 unicast-routing when the device must route IPv6 packets between interfaces; assigning one IPv6 address alone does not require the device to act as a router.

Use show ipv6 interface brief for addresses and line state, show ipv6 interface GigabitEthernet0/1 for global, link-local, and neighbor-discovery details, and show ipv6 route connected for the connected prefix. Then test an on-link neighbor before a remote destination. Exact interface names vary by platform.

Worked scenario

Two router interfaces should share 2001:db8:120:7::/64. Assign distinct host identifiers such as ::1/64 and ::2/64, enable both links, and verify their connected route. An address under 2001:db8:120:8::/64 is not on-link despite sharing most visible text.

Common traps

• Using :: twice.
• Dropping a nonzero group while compressing.
• Assuming every IPv6 prefix is /64.
• Looking for an IPv6 broadcast.
• Testing remote reachability before the directly connected neighbor.

Readiness checklist

• I can expand and compress addresses without changing value.
• I can compare addresses under a supplied prefix.
• I can configure and verify global and link-local state.
• I can distinguish connected reachability from remote routing.