Understanding IPv6

Author Joseph Davies Pages 544 Disk 1 Companion CD(s) Level Int/Adv Published 11/13/2002 ISBN 9780735612457 Price $29.99 To see this book's discounted price, select a reseller below.	More Information About the Book Table of Contents Sample Chapter Index Related Series Related Books About the Author Support: Book & CD Rate this book

Chapter 3: IPv6 Addressing continued

IPv6 Addresses for a Router

• A link-local address for each interface
• Additional unicast addresses for each interface (which could be a site-local address and one or multiple global addresses)
• The loopback address (::1) for the loopback interface

Additionally, the interfaces of an IPv6 router are assigned the following anycast addresses:

• A Subnet-Router anycast address for each subnet
• Additional anycast addresses (optional)

Additionally, the interfaces of an IPv6 router are listening for traffic on the following multicast addresses:

• The node-local scope all-nodes multicast address (FF01::1)
• The node-local scope all-routers multicast address (FF01::2)
• The link-local scope all-nodes multicast address (FF02::1)
• The link-local scope all-routers multicast address (FF02::2)
• The site-local scope all-routers multicast address (FF05::2)
• The solicited-node address for each unicast address
• The multicast addresses of joined groups

Subnetting the IPv6 Address Space

Subnetting for NLA IDs

For a global address allocated to a top-level aggregator, the first 16 bits of the address are fixed and correspond to the FP (set to 001) and the TLA ID (13 bits in length). The TLA ID is followed by the Res portion, which consists of 8 reserved bits set to 0. Therefore, for subnetting of the NLA ID portion of a global address, the first 24 bits are fixed. In a global address, the Res bits are never shown due to the suppression of leading zeros in IPv6 colon hexadecimal notation.

Subnetting the NLA ID portion of a global address requires a two-step procedure:

1. Determine the number of bits to be used for the subnetting.
2. Enumerate the new subnetted network prefixes.

The subnetting technique described here assumes that subnetting is done by dividing the 24-bit address space of the NLA ID using the high-order bits in the NLA ID that do not already have fixed values. While this method promotes hierarchical addressing and routing, it is not required. For example, you can also create a flat addressing space for the NLA ID by numbering the subnets from 0 to 16,777,215.

Step 1: Determining the Number of Subnetting Bits

The number of bits being used for subnetting determines the possible number of new subnetted network prefixes that can be allocated to portions of your network based on geographical, customer segment, or other divisions. In a hierarchical routing infrastructure, you need to determine how many network prefixes, and therefore how many bits, you need at each level in the hierarchy. The more bits you choose for the various levels of the hierarchy, the fewer bits you will have available to enumerate individual subnets in the last level of the hierarchy. The last level in the hierarchy is used to assign 48-bit prefixes to customer sites.

For example, a network designer at a large ISP decides to implement a two-level hierarchy reflecting a geographical/customer segment structure and uses 8 bits for the geographical level and 8 bits for the customer segment level. This means that each customer segment in each geographical location has only 8 bits of subnetting space left (24 – 8 – 8), or only 256 (= 2⁸) 48-bit prefixes per customer segment.

On any given level in the hierarchy, you will have a number of bits that are already fixed by the next level up in the hierarchy (f ), a number of bits used for subnetting at the current level in the hierarchy (s), and a number of bits remaining for the next level down in the hierarchy (r). At all times, f + s + r = 24. This relationship is shown in Figure 3-9.

Click to view graphic

Figure 3-9. The subnetting of an NLA ID

Step 2: Enumerating Subnetted Network Prefixes

Based on the number of bits used for subnetting, you must list the new subnetted network prefixes. There are two main approaches:

• Hexadecimal — Enumerate new subnetted network prefixes by using hexadecimal representations of the NLA ID and increment.
• Decimal — Enumerate new subnetted network prefixes by using decimal representations of the NLA ID and increment. The decimal subnetting technique is included here for those who are more comfortable dealing with decimal numbers (Base¹⁰).

Either method produces the same result: an enumerated list of subnetted network prefixes.

Creating the enumerated list of subnetted network prefixes by using the hexadecimal method

1. Based on s (the number of bits chosen for subnetting), and m (the prefix length of the network prefix being subnetted), calculate the following:

    f = m - 24

f is the number of bits within the NLA ID that are already fixed.

    n = 2^s

n is the number of network prefixes that are obtained.

    i = 2^24-(f+s)

i is the incremental value between each successive NLA ID expressed in hexadecimal form.

    l = 24 + f + s

l is the prefix length of the new subnetted network prefixes.

D = decimal representation of F

2. Create a three-column table with n entries. The first column is the network prefix number (starting with 1), the second column is the value of F (the hexadecimal representation of the NLA ID), and the third column is the new subnetted network prefix.
3. In the first table entry, the entry for the NLA ID column is F and the subnetted network prefix is the original network prefix with the new prefix length. To obtain F, combine the last two hexadecimal digits of the second hexadecimal block with the four hexadecimal digits of the third hexadecimal block of the NLA ID being subnetted to form a 6-digit hexadecimal number. Remember to include zeros that may not be present due to leading zero suppression. For example, for the global address prefix 3000:4D:C00::/38, F is 0x4D0C00.
4. In the next table entry, for the NLA ID column, increase the value of F by i. For example, in the second table entry, the NLA ID is F + i.
5. For the subnetted network prefix column, convert the NLA ID into two separate 16-bit blocks in colon hexadecimal notation and place them after the 16-bit prefix to express the new subnetted network prefix. For example, for the second table entry, the subnetted network prefix is [16-bit prefix]:[F ? i (expressed in colon hexadecimal notation)]::/l.
6. Repeat steps 4 and 5 until the table is complete.

For example, to perform a 3-bit subnetting of the global network prefix 3000:4D:C00::/38, we first calculate the values of the number of prefixes, the increment, and the new prefix length. Our starting values are F = 0x4D0C00, s = 3, and f = 38 – 24 = 14. The number of prefixes is 8 (n = 2³). The increment is 0x80 (i = 2^24-(14+3) = 128 = 0x80). The new prefix length is 41 (l = 38 + 3).

Next, we construct a table with 8 entries. The subnetted network prefix for network prefix 1 is 3000:4D:C00::/41. Additional entries in the table are successive increments of i in the NLA ID portion of the network prefix, as shown in Table 3-4.

Table 3-4. The Hexadecimal Subnetting Technique for Network Prefix 3000:4D:C00::/38

Network Prefix Number	NLA ID (hexadecimal)	Subnetted Network Prefix
1	4D0C00	3000:4D:C00::/41
2	4D0C80	3000:4D:C80::/41
3	4D0D00	3000:4D:D00::/41
4	4D0D80	3000:4D:D80::/41
5	4D0E00	3000:4D:E00::/41
6	4D0E80	3000:4D:E80::/41
7	4D0F00	3000:4D:F00::/41
8	4D0F80	3000:4D:F80::/41

Creating the enumerated list of subnetted network prefixes using the decimal method

1. Based on s (the number of bits chosen for subnetting), and m (the prefix length of the network prefix being subnetted), and F (the hexadecimal value of the NLA ID being subnetted), calculate the following:

    f = m - 24

f is the number of bits within the NLA ID that are already fixed.

    n = 2^s

n is the number of network prefixes that are obtained.

    i = 2^24-(f+s)

i is the incremental value between each successive NLA ID expressed in decimal form.

    l = 24 + f + s

l is the prefix length of the new subnetted network prefixes.

D = decimal representation of F

2. Create a four-column table with n entries. The first column is the network prefix number (starting with 1), the second column is the decimal representation of the NLA ID portion of the new subnetted network prefix, the third column is the hexadecimal representation of the NLA ID portion of the new subnetted network prefix, and the fourth column is the new subnetted network prefix.
3. In the first table entry, the decimal representation of the NLA ID is D, the hexadecimal representation of the NLA ID is F, and the subnetted network prefix is the original network prefix with the new prefix length.
4. In the next table entry, for the second column, increase the value of the decimal representation of the NLA ID by i. For example, in the second table entry, the decimal representation of the subnet ID is D + i.
5. For the third column, convert the decimal representation of the NLA ID to hexadecimal.
6. For the fourth column, convert the NLA ID into two separate 16-bit blocks in colon hexadecimal notation and place them after the 16-bit prefix to express the new subnetted network prefix. For example, for the second table entry, the subnetted network prefix is [16-bit prefix]:[F + i (expressed in colon hexadecimal notation)]::/l.

7. Repeat steps 4 through 6 until the table is complete.

For example, to perform a 3-bit subnetting of the global network prefix 3000:4D:C00::/38, we first calculate the values of the number of prefixes, the increment, and the new prefix length. Our starting values are F = 0x4D0C00, s = 3, and f = 38 - 24 = 14. The number of prefixes is 8 (n = 2³). The increment is 128 (i = 2^24-(14+3) = 128). The new prefix length is 41 (l = 38 + 3). The decimal representation of the starting NLA ID is 5049344 (D = 0x4D0C00 = 5049344).

Next, we construct a table with 8 entries. The subnetted network prefix for network prefix 1 is 3000:4D:C00::/41. Additional entries in the table are successive increments of i in the NLA ID portion of the network prefix, as shown in Table 3-5.

Table 3-5. The Decimal Subnetting Technique for Network Prefix 3000:4D:C00::/38

Network Prefix Number	Decimal Representation of NLA ID	Hexadecimal Representation of NLA ID	Subnetted Network Prefix
1	5049344	4D0C00	3000:4D:C00::/41
2	5049472	4D0C80	3000:4D:C80::/41
3	5049600	4D0D00	3000:4D:D00::/41
4	5049728	4D0D80	3000:4D:D80::/41
5	5049856	4D0E00	3000:4D:E00::/41
6	5049984	4D0E80	3000:4D:E80::/41
7	5050112	4D0F00	3000:4D:F00::/41
8	5050240	4D0F80	3000:4D:F80::/41

Subnetting for SLA IDs/Subnet IDs

In both cases, the first 48 bits of the address are fixed. For the global address, the first 48 bits are fixed and allocated by an ISP and correspond to the TLA and NLA ID portions of the global address. For the site-local address, the first 48 bits are fixed at FEC0::/48. In the discussion that follows, the term subnet ID refers to either the SLA ID portion of the global address or the Subnet ID portion of a site-local address.

Subnetting the subnet ID portion of a global or site-local address space requires a two-step procedure:

1. Determine the number of bits to be used for the subnetting.
2. Enumerate the new subnetted network prefixes.

The subnetting technique described here assumes that subnetting is done by dividing the 16-bit address space of the subnet ID using the high-order bits in the subnet ID. While this method promotes hierarchical addressing and routing, it is not required. For example, in a small organization with a small number of subnets, you can also create a flat addressing space for the subnet ID by numbering the subnets starting at 0. 

As described in the "Local-Use Unicast Addresses" section of this chapter, you can use the same subnetting scheme and use the same subnet ID for both site-local and global address network prefixes.

Step 1: Determining the Number of Subnetting Bits

The number of bits being used for subnetting determines the possible number of new subnetted network prefixes that can be allocated to portions of your network based on geographical or departmental divisions. In a hierarchical routing infrastructure, you need to determine how many network prefixes, and therefore how many bits, you need at each level in the hierarchy. The more bits you choose for the various levels of the hierarchy, the fewer bits you will have available to enumerate individual subnets in the last level of the hierarchy.

For example, a network administrator decides to implement a two-level hierarchy reflecting a geographical/departmental structure and uses 4 bits for the geographical level and 6 bits for the departmental level. This means that each department in each geographical location has only 6 bits of subnetting space left (16 - 6 - 4), or only 64 (= 2⁶) subnets per department.

On any given level in the hierarchy, you will have a number of bits that are already fixed by the next level up in the hierarchy (f), a number of bits used for subnetting at the current level in the hierarchy (s), and a number of bits remaining for the next level down in the hierarchy (r). At all times, f + s + r = 16. This relationship is shown in Figure 3-10.

Click to view graphic

Figure 3-10. The subnetting of a Subnet ID

Step 2: Enumerating Subnetted Network Prefixes

Based on the number of bits used for subnetting, you must list the new subnetted network prefixes. There are two main approaches:

• Hexadecimal — Enumerate new subnetted network prefixes by using hexadecimal representations of the subnet ID and increment.
• Decimal — Enumerate new subnetted network prefixes by using decimal representations of the subnet ID and increment.

Either method produces the same result: an enumerated list of subnetted network prefixes.

Creating the enumerated list of subnetted network prefixes using the hexadecimal method

1. Based on s (the number of bits chosen for subnetting), m (the prefix length of the network prefix being subnetted), and F (the hexadecimal value of the subnet being subnetted), calculate the following:

    f = m - 48

f is the number of bits within the subnet ID that are already fixed.

    n = 2^s

n is the number of network prefixes that are obtained.

    i = 2^16-(f+s)

i is the incremental value between each successive subnet ID expressed in hexadecimal form.

    l = 48 + f + s

l is the prefix length of the new subnetted network prefixes.

2. Create a two-column table with n entries. The first column is the network prefix number (starting with 1) and the second column is the new subnetted network prefix.
3. In the first table entry, based on F, the hexadecimal value of the subnet ID being subnetted, the subnetted network prefix is [48-bit prefix]:F::/l.
4. In the next table entry, increase the value within the subnet ID portion of the site-local or global address by i. For example, in the second table entry, the subnetted prefix is [48-bit prefix]:F + i::/l.

5. Repeat step 4 until the table is complete.

For example, to perform a 3-bit subnetting of the site-local network prefix FEC0:0:0:C000::/51, we first calculate the values of the number of prefixes, the increment, and the new prefix length. Our starting values are F = 0xC000, s = 3, and f = 51 - 48 = 3. The number of prefixes is 8 (n = 2³). The increment is 0x400 (i = 2^16-(3+3) = 1024 = 0x400). The new prefix length is 54 (l = 48 + 3 + 3).

Next, we construct a table with 8 entries. The entry for the network prefix 1 is FEC0:0:0:C000::/54. Additional entries in the table are successive increments of i in the subnet ID portion of the network prefix, as shown in Table 3-6.

Table 3-6. The Hexadecimal Subnetting Technique for Network Prefix FEC0:0:0:C000::/51

Network Prefix Number	Subnetted Network Prefix
1	FEC0:0:0:C000::/54
2	FEC0:0:0:C400::/54
3	FEC0:0:0:C800::/54
4	FEC0:0:0:CC00::/54
5	FEC0:0:0:D000::/54
6	FEC0:0:0:D400::/54
7	FEC0:0:0:D800::/54
8	FEC0:0:0:DC00::/54

Creating the enumerated list of subnetted network prefixes using the decimal method

1. Based on s (the number of bits chosen for subnetting), and m (the prefix length of the network prefix being subnetted), and F (the hexadecimal value of the subnet ID being subnetted), calculate the following:

    f = m - 48

f is the number of bits within the subnet ID that are already fixed.

    n = 2^s

n is the number of network prefixes that are obtained.

    i = 2^16-(f+s)

i is the incremental value between each successive subnet ID.

    l = 48 + f + s

l is the prefix length of the new subnetted network prefixes.

D = decimal representation of F

2. Create a three-column table with n entries. The first column is the network prefix number (starting with 1), the second column is the decimal representation of the subnet ID portion of the new network prefix, and the third column is the new subnetted network prefix.
3. In the first table entry, the decimal representation of the subnet ID is D and the subnetted network prefix is [48-bit prefix]:F::/l.
4. In the next table entry, for the second column, increase the value of the decimal representation of the subnet ID by i. For example, in the second table entry, the decimal representation of the subnet ID is D + i.

5. For the third column, convert the decimal representation of the subnet ID to hexadecimal and construct the prefix from [48-bit prefix]:[subnet ID]::/l. For example, in the second table entry, the subnetted network prefix is [48-bit prefix]:[D + i (converted to hexadecimal)]::/l.

6. Repeat steps 4 and 5 until the table is complete.

For example, to perform a 3-bit subnetting of the site-local network prefix FEC0:0:0:C000::/51, we first calculate the values of the number of prefixes, the increment, the new prefix length, and the decimal representation of the starting subnet ID. Our starting values are F = 0xC000, s = 3, and f = 51 - 48 = 3. The number of prefixes is 8 (n = 2³). The increment is 1024 (i = 2^16-(3+3)). The new prefix length is 54 (l = 48 + 3 + 3). The decimal representation of the starting subnet ID is 49152 (D = 0xC000 = 49152).

Next, we construct a table with 8 entries. The entry for the network prefix 1 is 49152 and FEC0:0:0:C000::/54. Additional entries in the table are successive increments of i in the subnet ID portion of the network prefix, as shown in Table 3-7.

Table 3-7. The Decimal Subnetting Technique for Network Prefix FEC0:0:0:C000::/51

Network Prefix Number	Decimal Representation of Subnet ID	Subnetted Network Prefix
1	49152	FEC0:0:0:C000::/54
2	50176	FEC0:0:0:C400::/54
3	51200	FEC0:0:0:C800::/54
4	52224	FEC0:0:0:CC00::/54
5	53248	FEC0:0:0:D000::/54
6	54272	FEC0:0:0:D400::/54
7	55296	FEC0:0:0:D800::/54
8	56320	FEC0:0:0:DC00::/54

IPv6 Interface Identifiers

The ways in which an interface identifier is determined are the following:

• As defined in RFC 2373, all unicast addresses that use the prefixes 001 through 111 must also use a 64-bit interface identifier that is derived from the Extended Unique Identifier (EUI)-64 address. The 64-bit EUI-64 address is defined by the Institute of Electrical and Electronic Engineers (IEEE). EUI-64 addresses are either assigned to a network adapter or derived from IEEE 802 addresses.
• As defined in RFC 3041, it might have a temporarily assigned, randomly generated interface identifier to provide a level of anonymity.
• It is assigned during stateful address autoconfiguration (for example, via Dynamic Host Configuration Protocol version 6 (DHCPv6)). Stateful address autoconfiguration standards and protocols are in progress.
• As defined in RFC 2472, an interface identifier can be based on link-layer addresses or serial numbers, or randomly generated when configuring a Point-to-Point Protocol (PPP) interface and an EUI-64 address is not available.
• It is assigned during manual address configuration.

EUI-64 Address-based Interface Identifiers

IEEE 802 Addresses

Network adapters for common LAN technologies such as Ethernet, Token Ring, and Fiber Data Distributed Interface (FDDI) use a 48-bit address called an IEEE 802 address. It consists of a 24-bit company ID (also called the manufacturer ID) and a 24-bit extension ID (also called the board ID). The combination of the company ID, which is uniquely assigned to each manufacturer of network adapters, and the extension ID, which is uniquely assigned to each network adapter at the time of manufacture, produces a globally unique 48-bit address. This 48-bit address is also called the physical, hardware, or media access control (MAC) address.

Figure 3-11 shows the structure of the 48-bit IEEE 802 address for Ethernet.

Click to view graphic

Figure 3-11. The structure of the 48-bit IEEE 802 address for Ethernet

Defined bits within the IEEE 802 address for Ethernet are:

Universal/Local (U/L) — The next-to-the low-order bit in the first byte is usedto indicate whether the address is universally or locally administered. If the U/L bit is set to 0, the IEEE (through the designation of a unique company ID) has administered the address. If the U/L bit is set to 1, the address is locally administered. In this case, the network administrator has overridden the manufactured address and specified a different address. The U/L bit is designated by the u in Figure 3-11.

Individual/Group (I/G) — The low-order bit of the first byte is used to indicate whether the address is an individual address (unicast) or a group address (multicast). When set to 0, the address is a unicast address. When set to 1, the address is a multicast address. The I/G bit is designated by the g in Figure 3-11.

For a typical 802.x network adapter address, both the U/L and I/G bits are set to 0, corresponding to a universally administered, unicast MAC address.

IEEE EUI-64 Addresses

The IEEE EUI-64 address represents a new standard for network interface addressing. The company ID is still 24-bits long, but the extension ID is 40 bits, creating a much larger address space for a network adapter manufacturer. The EUI-64 address uses the U/L and I/G bits in the same way as the IEEE 802 address.

Figure 3-12 shows the structure of the EUI-64 address.

Click to view graphic

Figure 3-12. The structure of the EUI-64 address

Mapping IEEE 802 Addresses to EUI-64 Addresses

To create an EUI-64 address from an IEEE 802 address, the 16 bits of 11111111 11111110 (0xFFFE) are inserted into the IEEE 802 address between the company ID and the extension ID, as shown in Figure 3-13.

Click to view graphic

Figure 3-13. The mapping of IEEE 802 addresses to EUI-64 addresses

Obtaining Interface Identifiers for IPv6 Addresses

To obtain the 64-bit interface identifier for IPv6 unicast addresses, the U/L bit in the EUI-64 address is complemented (if it is a 1 in the EUI-64 address, it is set to 0; and if it is a 0 in the EUI-64 address, it is set to 1).

The main reason for complementing the U/L bit is to provide greater compressibility of locally administered EUI-64 addresses. It is common practice when assigning locally administered addresses to number them in a simple way. For example, on a point-to-point link, you may assign one interface on the link the locally administered EUI-64 address of 02-00-00-00-00-00-00-01 and the other interface the locally administered EUI-64 address of 02-00-00-00-00-00-00-02. If the U/L bit is not complemented, the corresponding link-local addresses for these two interfaces become FE80::200:0:0:1 and FE80::200:0:0:2. By complementing the U/L bit, the corresponding link-local addresses for these two interfaces become FE80::1 and FE80::2.

Figure 3-14 shows the conversion of an EUI-64 address to an IPv6 interface identifier.

Click to view graphic

Figure 3-14. The conversion of an EUI-64 address to an IPv6 interface identifier

Converting IEEE 802 Addresses to IPv6 Interface Identifiers

To obtain an IPv6 interface identifier from an IEEE 802 address, you must first map the IEEE 802 address to an EUI-64 address, and then complement the U/L bit. Figure 3-15 shows this conversion process for a universally administered, unicast IEEE 802 address.

Click to view graphic

Figure 3-15. The conversion of an IEEE 802 address to an IPv6 interface identifier

IEEE 802 Address Conversion Example

Host A has the Ethernet MAC address of 00-AA-00-3F-2A-1C. First, it is converted to EUI-64 format by inserting FF-FE between the third and fourth bytes, yielding 00-AA-00-FF-FE-3F-2A-1C. Then, the U/L bit, which is the seventh bit in the first byte, is complemented. The first byte in binary form is 00000000. When the seventh bit is complemented, it becomes 00000010 (0x02). The final result is 02-AA-00-FF-FE-3F-2A-1C which, when converted to colon hexadecimal notation, becomes the interface identifier 2AA:FF:FE3F: 2A1C. As a result, the link-local address that corresponds to the network adapter with the MAC address of 00-AA-00-3F-2A-1C is FE80::2AA:FF:FE3F:2A1C.

Temporary Address Interface Identifiers

For IPv6-based dial-up connections, the user is assigned a 64-bit prefix, at the time of connection, by using router discovery, an exchange of Router Solicitation and Router Advertisement messages. If the interface identifier is always based on the EUI-64 address (as derived from the static IEEE 802 address), it is possible to identify the traffic of a specific node regardless of the prefix assigned at the time of connection. The use of the same 64-bit interface identifier allows identification of a user's traffic whether they are accessing the Internet from home or from work. This makes it easy for Internet merchants and malicious users to track a specific user and their use of the Internet.

To address this concern to provide the same level of anonymity as that provided with IPv4, an alternative derivation of the IPv6 interface identifier that is randomly generated and changes over time is discussed in RFC 3041.

The initial interface identifier is generated using random number techniques. For IPv6 systems that do not have the ability to store any history information for generating future values of the interface identifier, a new random interface identifier is generated each time the IPv6 protocol is initialized. For IPv6 systems that do have storage capabilities, a history value is stored and when the IPv6 protocol is initialized, a new interface identifier is created through the following process:

1. Retrieve the history value from storage and append the interface identifier based on the EUI-64 address of the adapter.
2. Compute the Message Digest-5 (MD5) hash over the quantity in step 1. The MD5 hash computation will produce a 128-bit value.
3. Store the low-order 64 bits of the MD5 hash computed in step 2 as the history value for the next computation of the interface identifier.
4. Take the high-order 64 bits of the MD5 hash computed in step 2 and set the seventh bit to zero. The seventh bit corresponds to the U/L bit, which, when set to 0, indicates a locally administered interface identifier. The result is the interface identifier.

The resulting IPv6 address, based on this random interface identifier, is known as a temporary address. Temporary addresses are generated for public address prefixes that use stateless address autoconfiguration. Temporary addresses are used for the lower of the following values of the valid and preferred lifetimes:

• The lifetimes included in the Prefix Information option in the received Router Advertisement message.
• Local default values of 1 week for valid lifetime and 1 day for preferred lifetime.

After the temporary address valid lifetime expires, a new interface identifier and temporary address is generated. For more information about router discovery, see Chapter 6, "Neighbor Discovery." For more information about stateless address autoconfiguration and valid and preferred lifetimes, see Chapter 8, "Address Autoconfiguration."

Mapping IPv6 Multicast Addresses to Ethernet Addresses

Click to view graphic

Figure 3-16. The mapping of IPv6 multicast addresses to Ethernet multicast addresses

Ethernet network adapters maintain a table of interesting destination MAC addresses. If an Ethernet frame with an interesting destination MAC address is received, it is passed to upper layers for additional processing. By default, this table contains the MAC-level broadcast address (0xFF-FF-FF-FF-FF-FF) and the unicast MAC address assigned to the adapter. To facilitate efficient delivery of multicast traffic, additional multicast destination addresses can be added or removed from the table. For every multicast address being listened to by the host, there is a corresponding entry in the table of interesting MAC addresses.

For example, an IPv6 host with the Ethernet MAC address of 00-AA-00-3F-2A-1C (link-local address of FE80::2AA:FF:FE3F:2A1C) adds the following multicast MAC addresses to the table of interesting destination MAC addresses on the Ethernet adapter:

• The address of 33-33-00-00-00-01, which corresponds to the link-local scope all-nodes multicast address of FF02::1.
• The address of 33-33-FF-3F-2A-1C, which corresponds to the solicited-node address of FF02::1:FF3F:2A1C. Remember that the solicited-node address is the prefix FF02::1:FF00:0/104 and the last 24 bits of the unicast IPv6 address.

Additional multicast addresses on which the host is listening are added and removed from the table as needed.

IPv4 Addresses and IPv6 Equivalents

Table 3-8. IPv4 Addressing Concepts and Their IPv6 Equivalents

IPv4 Address	IPv6 Address
Internet address classes	Not applicable in IPv6
Multicast addresses (224.0.0.0/4)	IPv6 multicast addresses (FF00::/8)
Broadcast addresses	Not applicable in IPv6
Unspecified address is 0.0.0.0	Unspecified address is ::
Loopback address is 127.0.0.1	Loopback address is ::1
Public IP addresses	Aggregatable global unicast addresses
Private IP addresses (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16)	Site-local addresses (FEC0::/48)
APIPA addresses (169.254.0.0/16)	Link-local addresses (FE80::/64)
Text representation: Dotted decimal notation	Text representation: Colon hexadecimal format with suppression of leading zeros and zero compression. IPv4-compatible addresses are expressed in dotted decimal notation.
Network bits representation: Subnet mask in dotted decimal notation or prefix length	Network bits representation: Prefix length notation only

References

RFC 2373 — "IP Version 6 Addressing Architecture"

RFC 2472 — "IP Version 6 over PPP"

RFC 3041 — "Privacy Extensions for Stateless Address Autoconfiguration in IPv6"

Testing for Understanding

1. Why is the IPv6 address length 128 bits?
2. Define the Format Prefixes (FPs) for commonly used unicast addresses.
3. Express FEC0:0000:0000:0001:02AA:0000:0000:0007A more efficiently.
4. How many bits are expressed by "::" in the addresses 3341::1:2AA: 9FF:FE56:24DC and FF02::2?
5. Describe the difference between unicast, multicast, and anycast addresses in terms of a host sending packets to zero or more interfaces.
6. Why are no broadcast addresses defined for IPv6?
7. Define the structure, including field sizes, of the aggregatable global unicast address.
8. Define the scope for each of the different types of typically used unicast addresses.
9. Explain how global and site-local addressing can share the same subnetting infrastructure within an organization.
10. Define the structure, including field sizes, of the multicast address.
11. Why does RFC 2373 recommend using only the last 32 bits of the IPv6 multicast address for the multicast group ID?
12. Explain how the solicited-node multicast address acts as a pseudo-unicast address.
13. How do routers know the nearest location of an anycast group member?
14. Perform a 4-bit subnetting on the site-local prefix FEC0:0:0:3D80::/57.
15. What is the IPv6 interface identifier for the universally administered, unicast IEEE 802 address of 0C-1C-09-A8-F9-CE? What is the corresponding link-local address? What is the corresponding solicited-node multicast address?
16. What is the IPv6 interface identifier for the locally administered, unicast EUI-64 address of 02-00-00-00-00-00-00-09? What is the corresponding link-local address?
17. What is the site-local scope multicast address corresponding to the Ethernet multicast MAC address of 33-33-00-0A-4F-11?
18. For each type of address, identify how the address begins in colon hexadecimal notation.

Type of Address	Begins with.
Link-local unicast address	FE80
Site-local unicast address
Global address
Multicast address
Link-local scope multicast address
Site-local scope multicast address
Solicited-node multicast address
IPv4-compatible address
IPv4-mapped address
6to4 address

Last Updated: October 28, 2002

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