In the IPv6 world, there is no such thing as broadcast traffic because it is all multicast traffic—no more broadcast storms. IPv6 multicast traffic has a prefix of FF00::/8 and is used as an identifier for a set of interfaces that want to receive the same packets. This is very similar to the way multi-cast works in the IPv4 world, with one exception. IPv6 multimulti-cast traffic can be limited to a certain scope. The octet after the initial 0xFF prefix defines the public/private nature and scope of the multicast address. The first nibble of the octet determines if this is a transient (0001) or permanent (0000) multicast address, with the first three bits always set to 0. A permanent multicast address is a well-known or IANA-assigned address. A transient address is locally assigned. The second nibble determines the scope of the multicast address and can be one of the following:
Interface-local, for loopback multicast transmissions only (0001–1)
Link-local (0010–2)
Subnet-local (0011–3)
Admin-local (0100–4)
Site-local (0101–5)
Organization-local (1000–8)
Global (1110–E)
The remaining 112 bits are used for the multicast group ID. This means that you can have millions of multicast groups. The following are the special reserved multicast addresses used to identify specific functions:
FF01::1—All nodes within the interface-local scope (only within this device)
FF02::1—All nodes on a local link (link-local scope)
FF01::2—All routers within the interface-local scope
FF02::2—All routers on a local link
FF05::2—All routers in the site (site-local scope)
FF02::1:FFXX:XXXX—Solicited-node multicast address, where XX:XXXX is the lower-order 24 bits of the IPv6 address of an interface.
The TTL—time to live—value is not used in IPv6 multicast to define scope.
Exam Essentials 73
The solicited-node multicast addresses are used in neighbor solicitation messages to assist with neighbor discovery. An IPv6 node must join the associated solicited-node multicast group for every unicast or anycast address assigned. Neighbor solicitation messages are not covered because they are beyond the scope of this study guide.
Summary
IP addresses can be separated into Classes A, B, C, D, and E. Class D is used for multicast traffic, and Class E is currently not being used. The first octet identifies to which class it belongs:
Class A addresses have first octets in the range from 0 to 127, Class B addresses have first octets from 128 to 191, and Class C addresses have first octets from 192 to 223. CIDR is used to easily identify the subnet mask of an IP address with slash notation, as well as to allow ISPs to assign non-classful address space to customers, thus reducing wasted addresses. VLSM is used to allow a network to be variably subnetted to make more efficient use of the IP addresses available.
Some routing protocols allow for VLSM (e.g., RIPv2 and EIGRP) because they transmit the mask of the network within the routing update.
Route summarization reduces the number of routes needed to represent a set of networks.
This preserves the resources, such as memory and processor cycles, on the routers in the net-work. When two or more subnets of the same classful network are separated by a different classful network, this makes the separated network discontiguous and will result in the inabil-ity to reduce the number of advertisements and subsequent routing table entries. For some routing protocols that automatically summarize routes, this can cause reachability problems, so you need to use a classless routing protocol and disable automatic summarization, if it is currently enabled.
With the global shortage of IPv4 address space, a new protocol has been introduced that will alleviate this problem without affecting end-to-end functionality. IPv6 not only greatly increases the number of IP addresses available, but it also brings improvements and new features to the IP protocol. These features are an expanded number of multicast addresses, the ability to natively support IPSec and QoS, and automatically determining the local subnet address without using DHCP by using the stateless auto-configuration process. IPv6 brings new features and concepts to networking that you will need to know.
Exam Essentials
Understand VLSM. Variable-length subnet masks enable a classful network to contain sub-networks of varying sizes. This allows a more efficient use of the network address space. For point-to-point links you can use a 30-bit or the new 31-bit mask, both of which allow for only two hosts, without requiring the same mask used on a LAN segment.
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Understand decimal-to-binary conversions. Decimal-to-binary conversion takes a base 10 number, the ones we are used to seeing, and converts it into a base 2 number, which is a series of 1s and 0s. This is important to know, because this base 2 number is the electronic basis for the network, subnet, and host portions of an IP address.
Understand CIDR notation. Classless interdomain routing started out as a way for service providers to offer customers address space more suited to their business needs without the constraints of classful boundaries. This process leads to less waste of IP addresses and allows continued use of IPv4, long after experts predicted we would have to convert to IPv6. A handy side-effect of the CIDR movement is the CIDR notation that allows us to represent the subnet mask in a one- or two-character shorthand (/xx).
Know how to implement VLSM. In implementing VLSM, you take a given address space—
usually a classful network—and break it up to allow networks of different sizes. You should be able to determine the correct subnet mask for each network according to the number of addresses needed and allocate it in such a way as to allow the most flexibility for future growth.
Understand the concept of route summarization. Route summarization is the process of combining multiple network advertisements into a single advertisement that can represent the combined networks. This technique allows the upstream routers to carry fewer network advertisements and reduces the size of the routing tables. This promotes stability and reduces memory and processor overhead in the backbone routers.
Know the three types of IPv6 addresses. The three types of IPv6 addresses are unicast, any-cast, and multicast. Although unicast and anycast addresses are indistinguishable from each other (because they are allocated from the same range 2000::/3), multicast addresses are allocated from the FF00::/8 address range. The format of IPv6 addresses is different from the quad-octet dotted-decimal IPv4 format, because each 16-bit field is represented by four hexadecimal numbers separated by a colon (:). There are eight of these fields in the 128-bit IPv6 address, and they can be compressed by deleting leading 0s. You can further compress an IPv6 address by substituting double colons (::) for a consecutive series of leading 0s, but this can appear only once in the address.
Understand the types of unicast and multicast addresses. Site-local addresses are a type of IPv6 unicast address, and they are used when you are not going to be connecting the network to the global Internet. Link-local addresses are another type of IPv6 unicast address, but unlike site-local addresses, link-site-local addresses are significant only to the site-local subnet. There are millions of IPv6 multicast addresses, and they can be scoped to limit the range the packets will travel. The fol-lowing are the available scopes listed from the smallest to the largest ranges: interface-local, link-local, subnet-link-local, admin-link-local, site-link-local, organization-link-local, and global.