Category: TCP/IP

Throughout this section there have been clues as to how hosts, and specifically interfaces, obtain their IPv6 addresses. The three methods by which an IPv6 host can obtain an IP address include stateless autoconfiguration, statefulautoconfiguration, or manually.
Stateless autoconfiguration is the easiest IPv6 address allocation method available. When used, stateless autoconfiguration uses the network prefix information contained in router advertisements as the first 64 bits of its addresses, and then appends its MAC address in EUI-64 format as the interface portion. This method is especially useful in environment where a DHCP server is neither configured nor present. On local networks without a router, a host using stateless autoconfiguration will use the link local network prefix and append to this its EUI-64 format MAC address.

For a higher degree on control over which addresses IPv6 interfaces use, statefulautoconfiguration can use another addressing method. When an IPv6 node sends out its router solicitation message at startup, the router can be configured to include whether a DHCP server should be used in its reply. If a DHCP server should be used, the node will used attempt to find a DHCP server through the use of multicasts. This is again an improvement over IPv4, where clients attempting to lease an IP address from a DHCP server use broadcast messages.

Finally, IPv6 addresses can also be configured manually. While generally not suggested for individual hosts, certain network nodes (such as routers) will require explicit configuration. Given the length and complexity of IPv6 addresses, it is generally best to use either stateful or stateless autoconfiguration for hosts to reduce potential errors and keep things simple.

Note: You may be curious about how DNS works in an IPv6 environment. Not surprisingly, the method is very similar to DNS in IPv4. However, when a host is attempting to obtain the IPv6 address associated with a fully qualified domain name (FQDN) or hostname, it sends a DNS query looking for the AAAA record associated with the host, rather than the standard A record used to resolve IPv4 addresses.

On an IPv6 network, a number of important functions happen using discovery processes. These include the discovery of neighboring devices and routers, as well as the maximum transmission unit (MTU) that is supported between a source and destination host. Some of these concepts are similar to ones found on an IPv4 network, while others represent new ways of dealing with traditional IPv4 configuration issues.

Neighbor discovery is the process by which an IPv6 node discovers the link-layer address of systems that it needs to communicate with on the local subnet, and is the method by which a node keeps track of local routers. This neighbor discovery process uses a new version of ICMP – ICMPv6. Ultimately, multicasts and anycasts are used for neighbor discovery functions on an IPv6 network. For example, recall the ARP function on an IPv4 network; a host would send out a broadcast requesting that the host with the specified IP send back its MAC address. In IPv6, ICMP multicasts are used to send out a request looking for the link-layer address associated with a known IPv6 address. This helps to reduce some of the traditional issues associated with broadcast traffic negatively impacting network performance.

Router discovery is a feature of IPv6 that allows an IPv6 node to discover the routers connected to its local link network. Although a similar feature exists in IPv4, it is rarely used with most administrators relying upon manually configured gateway addresses instead. Two main types of router discovery messages are used on IPv6 networks – router solicitations, and router advertisements. A router advertisement is a multicast message periodically sent by an IPv6 router that allows a host to gather valuable information about the network. For example, a router advertisement could contain information about the address configuration method that should be used, the IP prefixes in use on the network (or any changes to them), which router should be considered the default router, and more. Router advertisements will be looked at in more detail in the section on addressing hosts.

In contrast, a node sends out a router solicitation method when it does not have a configured IP address at start up. The purpose of this multicast is to gather information about how the node should be configured, but without the need to wait for the next router advertisement. For example, the result of a router solicitation message could be a reply from a local router specifying that the host should auto-configure its IPv6 address, or perhaps that it should use DHCP instead.

Another very important discovery process on IPv6 networks is maximum transmission unit (MTU) discovery. In previous articles in the CCNA series, we looked at how a router was capable of fragmenting an IPv4 packet when the next network on the path to a destination used a smaller MTU size – for example, when forwarding data from a Token Ring segment to an Ethernet segment. While this method helped to avoid some of the issues associated with interconnecting different network types, it also slowed down the communication process, since a router was not only responsible for reframing and making forwarding decisions, but also fragmenting packets and subsequently reframing them all as well. In IPv6, routers no longer fragment any packets. Instead, the sending node uses a process referred to as MTU discovery to determine the larger possible MTU that is supported between itself and the destination host. If any fragmentation needs to take place, it must be done on the sending node – IPv6 routers stay out of this process completely, leading to greater routing efficiency.

To understand how MTU discovery works, consider the figure below. In it, we see a source host attempting to discover the biggest MTU possible between itself and the destination. In this case, the MTU between Host A and its local router is 1500 bytes, the MTU between Routers A and B is 1400, and the MTU between Router B and the destination is 1200.

Figure

To discover the MTU, Host A will sent out a packet to Host B attempting to use its 1500 byte MTU. At Router A, an ICMPv6 error message (packet too big) will be sent back saying that an MTU of 1400 should be used. Host A will then send out another packet with an MTU of 1400, which will be designated as too big by Router B, with a maximum MTU of 1280 specified. Since Host B is connected to a network with an MTU of 1280, Host A now knows that this is the MTU that it should use to communicate with Host B. Although this process may seen cumbersome, it’s worth noting that IPv6 specifies a minimum MTU size of 1280 bytes, and that 1500 is usually the default MTU configured on most internetworking equipment.

Must like the reserved Class D address space in IPv4, IPv6 dedicates some of its address space to multicast traffic, albeit a much larger portion. If you recall, a multicast transmission is one in which a single transmission is received by many systems, or a one-to-many technique. In IPv6, multicasts use the prefix FF00::/8. Common examples of multicast addresses used in IPv6 include the destination address FF02::1, which is used to send a multicast to all hosts on a given subnet. Similarly, the multicast address FF02::2 is used to communicate with all routers on a subnet. Later in this series you’ll learn more about how some routing protocols use multicasts to facilitate inter-router communication.

IPv6 also defines an entirely new type of address and transmission, known as an “anycast”. Simply put, an anycast address is a standard IPv6 global address that is assigned to a number of different interfaces on different systems. When a packet is destined for an anycast address, the “closest” device to the sender will process the packet. In this case, the concept of “closest” is defined by the routing protocols in use on the network. At this time, anycast addresses can only be used as a destination address, cannot be used as a source address, and are only assigned to routers. A common use in IPv6 is to apply the same anycast address to all routers interfaces that connect to the same subnet. The potential uses of anycast transmission methods are being explored further by the IETF, and the technique is already finding its way into new technologies like Mobile IP.

Like in the world of IPv4, a unicast transmission represents data meant for a single destination address only. However, IPv6 uses a few different types of unicast addresses for different purposes. These include global, site-local, link-local, and IPv4-mapped IPv6 addresses. Each is outlined below.

Global Unicast Address. Very similar in function to an IPv4 unicast address such as 131.107.1.100, these addresses include a global routing prefix, a subnet ID, and an interface ID as outlined earlier.

Site-Local Unicast Address. Very similar in function to the IPv4 private address space that includes ranges like 10.0.0.0/8, these addresses are meant for internal communications and are not routable on the public Internet. Site-local addresses start with the prefix FEC0::/10, and then include the appropriate subnet ID and interface ID as outlined earlier.

Link-Local Unicast Address. For certain communications that are meant to stay within a given broadcast domain, IPv6 uses link-local addresses. These addresses are used for features like stateless autoconfiguration, which will be looked at shortly. Link-local addresses start with the prefix FE80::/10, and then include an interface ID. Note that since these addresses never communicate outside of their local subnet, the subnet ID is not included.

IPv4-mapped IPv6 Address. For environments that are transitioning between IPv4 and IPv6, IPv6 provides another type of unicast address known as an IPv4-mapped IPv6 address. This addressing method is used on systems running both an IPv4 and IPv6 protocol stack. When used, a system will include its current 32-bit IPv4 address in the low-order bits of an IPv6 address, preceded by 16 bits set to FFFF, and the remaining bits set to 0. For example, a host with the IPv4 address 131.107.1.100 would use the address of 0:0:0:0:0:FFFF:131.107.1.100.