This chapter covers IPv6 addressing and address types, a critical topic for the CompTIA Network+ N10-009 exam. IPv6 is essential because the world has run out of public IPv4 addresses, and networks increasingly require IPv6. Approximately 10–15% of exam questions touch IPv6 addressing, including address format, types (unicast, multicast, anycast), and configuration. Mastering this chapter will help you eliminate wrong answers on exam questions about address scopes, EUI-64, and SLAAC.
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Think of IPv4 as a postal system where addresses are limited to a few million homes (4.3 billion), and many neighborhoods (private IPs) share a single public mailbox (NAT). IPv6 is like a new postal system that gives every grain of sand on Earth its own unique address. The 128-bit address is like a global address format: the first 64 bits (the network prefix) are like the country, state, city, and street — assigned by the postal authority (ISP) — and the last 64 bits (the interface ID) are like the house number, often derived from the MAC address (like a unique building ID). Link-local addresses (fe80::/10) are like a temporary address used only within a single neighborhood; you cannot mail a letter using only a local address to someone in another city. Multicast (ff00::/8) is like a mass mailing to everyone on a certain street — one letter, many recipients. Anycast is like having multiple post offices with the same address; your letter automatically goes to the nearest one. Just as a postal worker doesn't need to remember every address, routers use the prefix to forward packets without knowing every host. And just as you can write a letter without a return address (unspecified ::), the system still works.
What is IPv6 and Why Does It Exist?
IPv6 (Internet Protocol version 6) is the successor to IPv4, defined in RFC 2460. The primary driver for IPv6 is the exhaustion of IPv4 address space. IPv4 uses 32-bit addresses, providing approximately 4.3 billion unique addresses. With the explosion of internet-connected devices, this is insufficient. IPv6 uses 128-bit addresses, providing 2^128 (approximately 3.4×10^38) addresses — enough for every person on Earth to have billions of addresses.
IPv6 Address Format
An IPv6 address is 128 bits long, typically represented as eight groups of four hexadecimal digits separated by colons. For example:
2001:0db8:85a3:0000:0000:8a2e:0370:7334Rules for shortening:
1. Leading zeros in each group can be omitted. So 0000 becomes 0, 0370 becomes 370.
2. One or more consecutive groups of zeros can be replaced with a double colon (::), but this can only be done once in an address to avoid ambiguity.
Example: The address above can be shortened to:
2001:db8:85a3::8a2e:370:7334Address Types
IPv6 defines three main address types: unicast, multicast, and anycast. There is no broadcast in IPv6; broadcast functionality is replaced by multicast.
#### Unicast Addresses
A unicast address identifies a single interface. Packets sent to a unicast address are delivered to the specific interface identified by that address. Unicast addresses are further categorized:
Global Unicast Addresses (GUA): These are globally routable and unique on the internet. They are equivalent to public IPv4 addresses. The prefix is 2000::/3 (first three bits 001). Typically, ISPs assign a /48 or /56 prefix to a site, and the site subnetting uses /64 subnets. The structure of a GUA:
- Global Routing Prefix (48 bits): assigned by the ISP - Subnet ID (16 bits): used by the organization for subnetting - Interface ID (64 bits): identifies the interface on the subnet
Link-Local Addresses: Every IPv6-enabled interface automatically generates a link-local address. These addresses are only valid on the local link (subnet) and are not routable. The prefix is fe80::/10 (first 10 bits 1111111010). The interface ID is typically derived from the MAC address using EUI-64 or randomly generated (privacy extensions). Link-local addresses are used for neighbor discovery, router advertisements, and other link-local protocols.
Unique Local Addresses (ULA): These are intended for local communication within a site or organization, similar to private IPv4 addresses (RFC 1918). The prefix is fc00::/7 (first 7 bits 1111110). The fd prefix is commonly used for locally assigned ULAs. The structure: fd + 40-bit Global ID + 16-bit Subnet ID + 64-bit Interface ID. ULAs are not routable on the global internet.
Loopback Address: ::1 (equivalent to 127.0.0.1 in IPv4). Used to test the local TCP/IP stack.
Unspecified Address: :: (all zeros). Used as a source address when the device does not yet have an IPv6 address (e.g., during Duplicate Address Detection).
#### Multicast Addresses
Multicast addresses identify a group of interfaces. A packet sent to a multicast address is delivered to all interfaces in the group. The prefix is ff00::/8 (first 8 bits all 1s). The structure:
First 8 bits: ff
Next 4 bits: flags (e.g., 0 for permanent, 1 for transient)
Next 4 bits: scope (1=node-local, 2=link-local, 5=site-local, 8=organization-local, E=global)
Remaining 112 bits: group ID
Common well-known multicast addresses:
- ff02::1 — All nodes on the local link
- ff02::2 — All routers on the local link
- ff02::1:ff00:0000/104 — Solicited-node multicast address (used for Neighbor Solicitation)
#### Anycast Addresses
Anycast addresses are assigned to multiple interfaces (usually on different devices). A packet sent to an anycast address is delivered to the nearest interface (based on routing protocol metric). Anycast addresses are taken from the unicast address space; there is no separate prefix. They are configured by assigning the same unicast address to multiple interfaces and advertising it via routing protocols. Anycast is commonly used for DNS root servers and load balancing.
EUI-64 Interface ID Generation
By default, IPv6 hosts generate their interface ID (last 64 bits) from the MAC address using the EUI-64 format. The process:
1. Split the 48-bit MAC address into two 24-bit halves.
2. Insert fffe in the middle (16 bits).
3. Invert the 7th bit of the first byte (the Universal/Local bit).
Example: MAC 00:1A:2B:3C:4D:5E becomes interface ID 021A:2BFF:FE3C:4D5E.
- First byte 00 becomes 02 (bit 2 inverted, so bit 1? Actually, the 7th bit is the second least significant bit of the first byte; 00 is 00000000, invert bit 1 (0-based from left? Wait, careful: The 7th bit (counting from 0 as MSB) in the first byte is the bit that indicates U/L. For MAC, the first byte's least significant bit is the multicast bit, and the second least significant bit is the U/L bit. So for 00, binary 00000000, the U/L bit is the second least significant bit (bit 1). Inverting it gives 00000010 = 02.)
Neighbor Discovery Protocol (NDP)
NDP (RFC 4861) is the IPv6 equivalent of ARP and ICMP Router Discovery. It uses ICMPv6 messages (types 133–137). Key functions: - Router Solicitation (RS): Sent by hosts to discover routers. - Router Advertisement (RA): Sent by routers periodically or in response to RS. Contains prefix information, default gateway, and other configuration options. - Neighbor Solicitation (NS): Sent to resolve a neighbor's link-layer address or to verify reachability. - Neighbor Advertisement (NA): Response to NS, providing the link-layer address. - Redirect: Informs a host of a better next-hop router.
Stateless Address Autoconfiguration (SLAAC)
SLAAC (RFC 4862) allows a host to automatically configure an IPv6 address without a DHCP server. The process: 1. Host generates a link-local address (fe80::/10 + EUI-64). 2. Host performs Duplicate Address Detection (DAD) by sending a Neighbor Solicitation for its own address. If no response, the address is unique. 3. Host sends a Router Solicitation to discover routers. 4. Router replies with a Router Advertisement containing the prefix and other configuration. The host combines the prefix with its interface ID to form a global unicast address. 5. Host performs DAD again for the new address.
Address Scope
IPv6 addresses have defined scopes: - Interface-local: Loopback (::1) - Link-local: fe80::/10 — only valid on the local link - Site-local: Deprecated (formerly fec0::/10), replaced by ULA - Global: 2000::/3 — globally routable
Configuration Commands
On Cisco IOS, to configure an IPv6 address on an interface:
interface GigabitEthernet0/0
ipv6 address 2001:db8:1::1/64
ipv6 enableTo enable IPv6 routing:
ipv6 unicast-routingTo view IPv6 interface information:
show ipv6 interface brief
show ipv6 routeInteraction with Other Technologies
IPv6 can coexist with IPv4 in dual-stack configurations. Tunneling (e.g., 6to4, Teredo) allows IPv6 packets to traverse IPv4-only networks. DNS records for IPv6 use AAAA (quad-A) records. The exam may test knowledge of these transition mechanisms but focuses primarily on addressing.
Host generates link-local address
When an IPv6-enabled interface comes up, the host automatically creates a link-local address using the fe80::/10 prefix. The interface ID is typically derived from the MAC address using EUI-64 or randomly generated if privacy extensions are enabled. The host then assigns this address to the interface. This address is only valid on the local link and is used for neighbor discovery and other link-local communications.
Duplicate Address Detection (DAD)
Before using the link-local address, the host must ensure it is unique on the link. The host sends a Neighbor Solicitation (NS) message to the solicited-node multicast address corresponding to its tentative address. The NS contains the tentative address as the target. If another host responds with a Neighbor Advertisement (NA), the address is a duplicate, and the host must generate a new address. If no response within a few seconds (default: 1 second retry interval, 3 retries), the address is considered unique.
Host sends Router Solicitation (RS)
After DAD succeeds, the host sends a Router Solicitation message to the all-routers multicast address (ff02::2) to discover routers on the link. The RS is sent from the host's link-local address. This prompts routers to send Router Advertisements (RAs) immediately, rather than waiting for their periodic RA interval (typically 200 seconds).
Router sends Router Advertisement (RA)
Routers periodically send Router Advertisements (default every 200 seconds) or respond to RS messages. The RA includes the prefix(es) of the subnet (e.g., 2001:db8:1::/64), the default gateway (the router's link-local address), and other configuration parameters like hop limit and MTU. The RA also contains flags: M flag (Managed address configuration) indicates DHCPv6 is available, O flag (Other configuration) indicates DHCPv6 for other parameters. If both flags are 0, SLAAC is used.
Host configures global unicast address
Upon receiving the RA, the host uses the advertised prefix (e.g., 2001:db8:1::/64) and combines it with its interface ID to form a global unicast address. The host then performs DAD on this new address. If no duplicate is found, the address is assigned to the interface. The host also sets the default gateway to the router's link-local address from the RA. This completes SLAAC.
In an enterprise network, IPv6 deployment often starts with dual-stack, where both IPv4 and IPv6 are enabled on all devices. A common scenario is a corporate campus with multiple VLANs. The network engineer assigns a /48 global prefix from the ISP and subnets it into /64 subnets for each VLAN. For example, prefix 2001:db8:corp::/48 might be subnetted as 2001:db8:corp:1::/64 for the engineering VLAN, 2001:db8:corp:2::/64 for HR, etc. Routers are configured with IPv6 routing (OSPFv3 or EIGRP for IPv6) and send Router Advertisements with the appropriate prefix for each VLAN. Hosts use SLAAC to configure their addresses automatically.
A second scenario is a data center using anycast for DNS servers. The organization runs multiple DNS resolvers with the same IPv6 address (e.g., 2001:db8:dns::1). This address is advertised via BGP from multiple locations. When a client sends a DNS query to that address, BGP routes the packet to the nearest anycast server, improving latency and providing redundancy. If one server fails, traffic automatically goes to the next closest.
A third scenario involves cloud migration. When moving services to AWS or Azure, IPv6 support is crucial because many cloud providers charge for public IPv4 addresses. The engineer configures a /56 prefix from the cloud provider's IPv6 block, assigns /64 subnets to VPCs, and uses SLAAC or DHCPv6 for EC2 instances. Security groups and network ACLs must be updated to filter IPv6 traffic appropriately.
Common misconfigurations: Forgetting to enable ipv6 unicast-routing on routers, which stops RA generation. Using the wrong prefix length (e.g., /48 instead of /64 for a subnet) breaks SLAAC because SLAAC requires a /64. Not filtering ICMPv6 (especially Neighbor Discovery) can cause security issues. Also, mixing SLAAC and DHCPv6 without proper flag settings leads to confusion.
The N10-009 exam tests IPv6 addressing under Objective 1.4 (Given a scenario, configure and verify network settings). Expect 3–5 questions on IPv6 address types, format, and autoconfiguration. Key points the exam loves:
Address shortening: Be able to expand :: correctly. Common trap: Forgetting that :: can only be used once. For example, 2001::1:2:3:4 could be ambiguous if multiple zero groups exist. The exam asks which address is valid or to identify the full address.
Address types: Know the prefixes: GUA (2000::/3), link-local (fe80::/10), ULA (fc00::/7), multicast (ff00::/8). Trap: Many candidates confuse link-local with GUA because both are unicast. Link-local addresses are not routable; they are only for local link communication. If a question says "a host can use this address to communicate with another host on the same subnet but not across routers," the answer is link-local.
EUI-64: The exam tests the inversion of the 7th bit (U/L bit). Common wrong answer: Candidates think the bit is the multicast bit (first bit) or the last bit. Remember: The U/L bit is the second least significant bit of the first byte. For MAC 00:..., the EUI-64 starts with 02.
SLAAC vs DHCPv6: The exam asks which method is used based on RA flags. M=1 means DHCPv6 for addresses; O=1 means DHCPv6 for other config. If both are 0, SLAAC is used. Trap: Candidates think SLAAC always requires DHCPv6. It does not; SLAAC can provide all necessary info from RA.
Multicast scopes: Know the scope values: 2=link-local, 5=site-local, 8=organization-local, E=global. The solicited-node multicast address is ff02::1:ffXX:XXXX, where XX:XXXX is the last 24 bits of the target address. This is used for DAD and neighbor resolution.
Anycast: Understand that anycast addresses are taken from the unicast address space and are indistinguishable from unicast until routing is configured. Trap: Candidates think anycast has a special prefix; it does not.
No broadcast: IPv6 does not use broadcast. All broadcast functions are replaced by multicast. If an exam answer mentions "broadcast" in IPv6 context, it is likely wrong.
To eliminate wrong answers, focus on the mechanism: If an address is not routable, it must be link-local. If it starts with ff, it is multicast. If it is used for one-to-nearest, it is anycast.
Edge cases: The unspecified address :: is used as a source during DAD. The loopback address ::1 is for local testing. A host can have multiple IPv6 addresses (link-local + GUA + possibly ULA).
Numbers to memorize: /64 for SLAAC subnets, /10 for link-local, /8 for multicast, /3 for global unicast, /7 for ULA.
IPv6 address is 128 bits, represented as 8 groups of 4 hex digits; use :: to shorten one contiguous zero group.
Global unicast addresses start with 2000::/3; link-local with fe80::/10; unique local with fc00::/7; multicast with ff00::/8.
There is no broadcast in IPv6; use multicast ff02::1 for all-nodes and ff02::2 for all-routers.
EUI-64 inserts FFFE into MAC and inverts the 7th bit (U/L bit) of the first byte.
SLAAC uses Router Advertisements (ICMPv6 type 134) to provide prefix and default gateway; no DHCPv6 needed unless M flag is set.
Duplicate Address Detection (DAD) uses Neighbor Solicitation to the solicited-node multicast address.
Anycast addresses are taken from unicast space; packets go to the nearest interface that shares the address.
Link-local addresses are automatically generated and not routable; used for NDP and local communication.
The unspecified address :: is used as source during DAD; loopback is ::1.
Common exam trap: confusing link-local with global unicast; link-local is not routable.
IPv6 subnets for SLAAC must be /64; other prefix lengths break autoconfiguration.
Router Advertisement flags: M=1 means DHCPv6 for addresses; O=1 means DHCPv6 for other config; both 0 means SLAAC only.
These come up on the exam all the time. Here's how to tell them apart.
Global Unicast Address (GUA)
Prefix 2000::/3
Globally routable on the internet
Assigned by ISP or RIR
Equivalent to public IPv4 address
Must be unique globally
Unique Local Address (ULA)
Prefix fc00::/7, typically fd00::/8
Not routable on the internet
Locally assigned within organization
Equivalent to private IPv4 (RFC 1918)
Unique within organization but may be used by multiple orgs
SLAAC
Stateless; host generates its own address
Uses Router Advertisements
No server required
Provides only prefix and default gateway
No logging of address assignments
DHCPv6
Stateful; server assigns addresses
Uses DHCPv6 messages
Requires DHCPv6 server
Can provide DNS, domain, and other options
Logs address assignments
Mistake
IPv6 addresses are always 128 bits long, so every address has 32 hex digits.
Correct
While the address is 128 bits, the shortened form using :: can reduce the representation. The full address must have 32 hex digits if expanded, but the shortened form is valid and common.
Mistake
Link-local addresses are globally unique and routable.
Correct
Link-local addresses (fe80::/10) are only valid on the local link. Routers do not forward packets with link-local source or destination addresses. They are automatically generated and used for neighbor discovery.
Mistake
IPv6 uses broadcast addresses like IPv4.
Correct
IPv6 does not have broadcast addresses. All broadcast functionality (e.g., ARP) is replaced by multicast. The all-nodes multicast address ff02::1 serves a similar purpose to the IPv4 broadcast, but it is a multicast address.
Mistake
SLAAC requires a DHCPv6 server to assign addresses.
Correct
SLAAC (Stateless Address Autoconfiguration) does not require DHCPv6. The host generates its own address using Router Advertisements. DHCPv6 is optional and used only if the RA flags indicate (M flag set).
Mistake
The EUI-64 process simply inserts FFFE into the MAC address.
Correct
EUI-64 inserts FFFE and then inverts the 7th bit (U/L bit) of the first byte. For example, MAC 00:1A:2B:3C:4D:5E becomes interface ID 021A:2BFF:FE3C:4D5E, not 001A:2BFF:FE3C:4D5E.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
To shorten an IPv6 address, remove leading zeros in each group and replace one contiguous sequence of zero groups with ::. For example, 2001:0db8:0000:0000:0000:8a2e:0370:7334 becomes 2001:db8::8a2e:370:7334. You can only use :: once. If there are multiple zero groups, choose the longest sequence. If equal, use the first one. On the exam, be able to expand shortened addresses by inserting zeros until you have 8 groups.
Link-local addresses (fe80::/10) are automatically generated on every IPv6 interface and are only valid on the local link. They are not routable and are used for neighbor discovery, router advertisements, and other link-local protocols. Global unicast addresses (2000::/3) are globally routable and uniquely assigned. A host typically has both. On the exam, if a question says 'an address that works only on the same subnet,' the answer is link-local.
No, IPv6 uses Neighbor Discovery Protocol (NDP) instead of ARP. NDP uses ICMPv6 messages: Neighbor Solicitation (NS) and Neighbor Advertisement (NA). These messages are sent to solicited-node multicast addresses, not broadcast. This reduces network overhead compared to IPv4 ARP broadcasts.
The solicited-node multicast address is used in Duplicate Address Detection (DAD) and address resolution (replacing ARP). It is formed by taking the last 24 bits of the target IPv6 address and appending them to ff02::1:ff00:0/104. For example, for address 2001:db8::1:2:3:4, the solicited-node multicast is ff02::1:ff03:4. This limits multicast group membership to only those interested in that address.
No, SLAAC requires a /64 prefix because the interface ID is 64 bits. If the prefix length is not /64, the host cannot form a complete address using EUI-64 or random generation. For subnetting, you can assign longer prefixes (e.g., /48, /56) to sites, but each subnet must be /64. On the exam, if a question mentions SLAAC, the subnet must be /64.
SLAAC (Stateless Address Autoconfiguration) allows hosts to generate their own addresses using Router Advertisements. It does not require a server. DHCPv6 is a client-server protocol that assigns addresses and other configuration parameters. The RA flags (M and O) indicate whether DHCPv6 should be used. If M=0 and O=0, SLAAC only. If M=1, DHCPv6 for addresses. If O=1, DHCPv6 for other config (like DNS).
The double colon (::) is used to shorten an IPv6 address by replacing one contiguous sequence of one or more groups of zeros. It can only appear once in an address. For example, 2001:0:0:0:0:0:0:1 becomes 2001::1. This reduces the address length and makes it easier to read. On the exam, you may be asked to expand a shortened address.
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