Quality of Service and performance characterization of IPv6 relative to IPv4

II.4.2 IPv6 addressing notation

Since IPv6 addresses are 128 bits long (compared to IPv4's 32 bits), a different representation is required.

The preferred way of writing an IPv6 address is:

The IPv6 128-bit address is divided along 16-bit boundaries. Each 16-bit block is then converted to a 4-digit hexadecimal number [0-9, A-F], separated by colons. The resulting representation is called colon-hexadecimal. This is in contrast to the 32-bit IPv4 address represented in dotted-decimal format, divided along 8-bit boundaries, and then converted to its decimal [0-9] equivalent, separated by periods. ^15(*)

The following example shows a 128-bit IPv6 address in binary form [0-1]:

0010000111011010000000001101001100000000000000000010111100111011

0000001010101010000000001111111111111110001010001001110001011010

The following example shows this same address divided along 16-bit boundaries:

0010000111011010 0000000011010011 0000000000000000 0010111100111011

0000001010101010 0000000011111111 1111111000101000 1001110001011010

The following example shows each 16-bit block in the address converted to hexadecimal [0-9, A-F] and delimited with colons.

21DA:00D3:0000:2F3B:02AA:00FF:FE28:9C5A

The IPv6 address is case insensitive; this example can be also written like this:

21da:00D3:0000:2f3b:02AA:00ff:FE28:9C5A

IPv6 representation can be further simplified by removing the leading zeros within each 16-bit block. However, each block must have at least a single digit. The following example shows the address without the leading zeros:

21DA:D3:0:2F3B:2AA:FF:FE28:9C5A

IPv6 representation can also contain long sequences of zeros. In IPv6 addresses, a contiguous sequence of 16-bit blocks set to 0 in the colon-hexadecimal format can be compressed to :: (known as double-colon), Zero compression can be used only once in an address. The following example shows the address of compressing zeros:

The link-local address

FE80:0:0:0:2AA:FF:FE9A:4CA2

Can be compressed to

FE80::2AA:FF:FE9A:4CA2.

The biggest reduction is seen by the IPv6 local host address:

0000:0000:0000:0000:0000:0000:0000:0001 -> ::1

Zero compression enables to determine the number of 0 bits represented by each instance of a double-colon (::). To determine how many 0 bits are represented by the ::, count the number of blocks in the compressed address, subtract this number from 8, and then multiply the result by 16. The following example shows how to determine zero compression:

FF02::2

There are two blocks (the FF02 block and the 2 block). The number of bits expressed by the :: is 96 (96 = (8 - 2) × 16).

II.4.3 IPv6 prefixes notation

The prefix is the part of the address that indicates the bits that have fixed values or are the bits of the network identifier.

IPv6 prefix <1-128 bits> is very similar to the way IPv4 <1-32 bits> addresses are written in Classless Interdomain Routing (CIDR) notation and it is also commonly used for subnetted IPv4 addresses <IPv4 address / subnet mask>.

An IPv6 prefix is written in notation appends the prefix length, written as a number of bits with a slash, which leads to the following format:

<IPv6address/prefix length>

The prefix length specifies how many left-most bits of the address specify the prefix. This is another way of noting a subnet mask.

In IPv4 a subnet mask specifies the bits of the IPv4 address that belong to the network ID. The prefix is used to identify the subnet that an interface belongs to and is used by routers for forwarding.

The following explains how the prefix is interpreted as shown in the table below:

IPv6 prefix notation 2E78:DA53:12::/40^16(*)

Hexadecimal notation	Binary notation	Bits of prefix
2E78	0010 1110 0111 1000	16 bits
DA53	1101 1010 0101 0011	16 bits
12	0001 0010	8 bits
		Total =16+16+8= 40

Table 1:IPv6 prefix notation

Source: IPv6 essentials, O'REILLY, ISBN: 0-596-00125-8, page 28

* ¹⁵ http://msdn.microsoft.com/library/default.asp?url=/library/en-us/wcecomm5/html/wce50conipv6addresses.asp (June 02, 2006)

* ¹⁶ IPv6 essentials, O'REILLY, ISBN: 0-596-00125-8, page 26.