3.5 IPv6 Address and Prefix Formats

Reading IPv6 addresses

Cisco includes identifying IPv6 addresses and prefix formats in the CCST Networking objectives. IPv6 addresses are 128 bits long, written as eight groups of hexadecimal numbers separated by colons. A full address might look like 2001:0db8:0000:0000:0000:ff00:0042:8329. Each group represents 16 bits. Hexadecimal uses digits 0 through 9 and letters a through f, so 00ff is valid and 00gg is not.

IPv6 has two important shorthand rules. First, leading zeros inside a group can be removed. 0db8 becomes db8, and 0042 becomes 42. Second, one consecutive run of all-zero groups can be replaced with ::. The full example can become 2001:db8::ff00:42:8329. The :: can appear only once in an address, because using it twice would make the number of omitted zero groups ambiguous. For example, 2001:db8::1 is valid, but 2001::db8::1 is not valid.

IPv6 prefixes use slash notation just like CIDR. 2001:db8:10:20::/64 identifies a 64-bit network prefix. A host address might be 2001:db8:10:20::25/64. On most LANs, /64 is the standard subnet size because IPv6 neighbor discovery, stateless address autoconfiguration, and many design expectations assume a 64-bit interface identifier. Do not calculate IPv6 host counts the way you do small IPv4 subnets in everyday support; the address space is intentionally huge.

Common IPv6 address types are easy to recognize by prefix. Global unicast addresses are publicly routable and commonly start within 2000::/3, which includes addresses beginning with 2 or 3. Documentation examples often use 2001:db8::/32, which is reserved for examples and should not be used as real production space. Link-local addresses begin with fe80::/10. Every IPv6-enabled interface commonly has a link-local address, and routers use link-local addresses for local control traffic. Unique local addresses begin with fc00::/7, with fd commonly seen in labs and internal networks.

IPv6 address assignment can be static, stateful through DHCPv6, or automatic through SLAAC, which stands for stateless address autoconfiguration. In SLAAC, router advertisements tell hosts the prefix and other information, and the host forms its own address. Some networks use DHCPv6 for additional control or options, and some use both router advertisements and DHCPv6. For first-level support, the key point is that IPv6 clients may not depend on DHCP in exactly the same way IPv4 clients do.

IPv6 does not use broadcast. Instead, it uses multicast and Neighbor Discovery Protocol, or NDP. NDP performs jobs similar to ARP and router discovery in IPv4. When an IPv6 host needs the MAC address for a neighbor on the same link, it uses neighbor solicitation and neighbor advertisement messages. When it needs router information, it can learn from router advertisements. This is one reason blocking all ICMPv6 can break IPv6 even when basic addressing looks right.

IPv6 hosts often have multiple addresses at the same time: a link-local address, one or more global unicast or unique local addresses, and temporary privacy addresses. That is normal. When troubleshooting, identify which address was used for the failing connection and include the prefix length and interface. A ping to a link-local address may require an interface zone identifier, such as %en0 or %12, because the same link-local prefix exists on every interface.

A practical comparison helps: IPv4 192.168.10.25/24 and IPv6 2001:db8:10:20::25/64 both include an address and a prefix. The notation differs, but the technician question is similar: what network is this host on, what gateway or router does it use, and should the destination be local or routed?

Study Checkpoint

• Topic: 3.5 IPv6 Address and Prefix Formats.
• Verify the official Cisco concept before memorizing a shortcut.
• Practice the technician action: observe, document, test, fix when supported, or escalate.