Course contents

Chapter 3: IPv6 ConfigurationLesson 1 of 6

IPv4 vs IPv6

Address size, notation, header, fragmentation, broadcast and configuration compared, plus dual stack, tunnelling and translation.

Beginner · 8 min read

Technical definition

IPv6 is the version of the Internet Protocol that uses 128-bit addresses. It was designed to replace IPv4, whose 32-bit address space is too small, and it also simplifies the packet header and removes broadcast.

Why a second version exists

IPv4 has about 4.3 billion addresses (232), and the pool of new ones ran out years ago. Private addresses and NAT stretched it, but they add complexity and break end-to-end connectivity. IPv6 has 2128 addresses, roughly 3.4 × 1038, so every device can have a globally unique one.

IPv4 and IPv6 side by side
IPv4 and IPv6 side by sideFeature; IPv4; IPv6. Address length: 32 bits 128 bits Notation: Dotted decimal: 192.0.2.10 Hexadecimal groups: 2001:db8:1::10 Address count: About 4.3 billion About 3.4 × 10^38 Header: 20 bytes minimum, variable, with options and a checksum Fixed 40 bytes, no checksum, extension headers for extras Fragmentation: Routers or the sender may fragment Only the sender fragments; routers never do One to everyone: Broadcast No broadcast: multicast replaces it Finding a neighbour's MAC: ARP (broadcast) Neighbor Discovery over ICMPv6 (multicast) Address configuration: Manual or DHCP Manual, SLAAC or DHCPv6 Typical subnet: Sized to the hosts, such as /24 Always /64 on a LAN NAT: Widely used Not needed Local address on every interface: Only if DHCP fails (169.254.0.0/16) Always: a link-local address, fe80::/10FeatureIPv4IPv6Address length32 bits128 bitsNotationDotted decimal: 192.0.2.10Hexadecimal groups: 2001:db8:1::10Address countAbout 4.3 billionAbout 3.4 × 10^38Header20 bytes minimum, variable, withoptions and a checksumFixed 40 bytes, no checksum,extension headers for extrasFragmentationRouters or the sender may fragmentOnly the sender fragments; routersnever doOne to everyoneBroadcastNo broadcast: multicast replaces itFinding a neighbour'sMACARP (broadcast)Neighbor Discovery over ICMPv6(multicast)Address configurationManual or DHCPManual, SLAAC or DHCPv6Typical subnetSized to the hosts, such as /24Always /64 on a LANNATWidely usedNot neededLocal address on everyinterfaceOnly if DHCP fails (169.254.0.0/16)Always: a link-local address,fe80::/10

On small screens, swipe sideways or focus the diagram and use the arrow keys.

Read each row across. The biggest practical changes for a CCNA are the /64 subnet, multicast instead of broadcast and NDP instead of ARP.

What stays the same

The ideas carry over: a router forwards by longest prefix match, a host sends off-link traffic to its default gateway, and TCP and UDP sit on top unchanged. Routing protocols have IPv6 versions, such as OSPFv3 and MP-BGP. Learn the IPv4 forms first, then map each one across.

Living with both: three transition methods

How IPv4 and IPv6 coexist
How IPv4 and IPv6 coexistMethod; What it does; Where you meet it. Dual stack: Each device runs IPv4 and IPv6 at the same time Most enterprise and home networks today Tunnelling: Wraps IPv6 packets inside IPv4 so they cross an IPv4-only network Between IPv6 sites over an IPv4 provider Translation (NAT64 with DNS64): Converts between IPv6 and IPv4 so IPv6-only hosts reach IPv4 servers Mobile and IPv6-only networksMethodWhat it doesWhere you meet itDual stackEach device runs IPv4 and IPv6 at thesame timeMost enterprise and home networkstodayTunnellingWraps IPv6 packets inside IPv4 sothey cross an IPv4-only networkBetween IPv6 sites over an IPv4providerTranslation (NAT64 withDNS64)Converts between IPv6 and IPv4 soIPv6-only hosts reach IPv4 serversMobile and IPv6-only networks

On small screens, swipe sideways or focus the diagram and use the arrow keys.

Dual stack is the preferred method; the other two are used when a network cannot yet run both.

On a dual-stack host, the operating system prefers IPv6 when the destination has an AAAA record and a working path. Slow connections are often traced to a broken IPv6 path that the host tries first.

Why the CCNA cares

The exam expects you to read an IPv6 address, name its type, configure it, and know that neighbour discovery and the /64 rule replace ARP and host-count subnetting. The next lessons take each in turn: the header, subnetting, neighbour discovery and SLAAC.

Test yourself

Predict · scenario 1

A host has only 169.254.x.x in IPv4 but still reaches IPv6 sites. Which IPv6 address makes local communication possible without any server?

Predict · scenario 2

Which statement about fragmentation is correct for IPv6?

Predict · scenario 3

Which pair correctly maps an IPv4 function to its IPv6 replacement?

FAQ

Does IPv6 replace IPv4 completely?
Not yet. Most networks run both, called dual stack, and will for years. Devices that only speak IPv4 still need translation or a dual-stack gateway to reach IPv6-only services.
Is IPv6 more secure than IPv4?
Not by itself. IPsec support is part of the IPv6 specification, but it is not switched on automatically. IPv6 still needs firewalls, ACLs and the usual protections, and it adds its own risks such as rogue router advertisements.
Why does IPv6 not need NAT?
There are enough global addresses for every device, so each can have its own. NAT66 exists but is rarely used. A stateful firewall, not address translation, is what keeps unwanted traffic out.