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Chapter: Computer Networks : Network Layer

IPv6 Addresses

Despite all short-term solutions, such as classless addressing, Dynamic Host Configuration Protocol (DHCP) and NAT, address depletion is still a long-term problem for the Internet.

IPv6 Addresses


Despite all short-term solutions, such as classless addressing, Dynamic Host Configuration Protocol (DHCP) and NAT, address depletion is still a long-term problem for the Internet. This and other problems in the IP protocol itself, such as lack of accommodation for real-time audio and video transmission, and encryption and authentication of data for some applications, have been the motivation for IPv6.


1. Structure


An IPv6 address consists of 16 bytes (octets); it is 128 bits long. An IPv6 address is 128 bits long.


2. Hexadecimal Colon Notation


To make addresses more readable, IPv6 specifies hexadecimal colon notation. In this notation, 128 bits is divided into eight sections, each 2 bytes in length. Two bytes in hexadecimal notation requires four hexadecimal digits. Therefore, the address consists of 32 hexadecimal digits, with every four digits separated by a colon


3. Abbreviation


Although the IP address, even in hexadecimal format, is very long, many of the digits are zeros. In this case, we can abbreviate the address. The leading zeros of a section (four digits between two colons) can be omitted. Only the leading zeros can be dropped, not the trailing zeros.


Using this form of abbreviation, 0074 can be written as 74, 000F as F, and 0000 as 0.


Example 3.11

Expand the address 0:15:: 1:12:1213 to its original.




We first need to align the left side of the double colon to the left of the original pattern and the right side of the double colon to the right of the original pattern to find now many Os we need to replace the double colon.


xxxx: xxxx: xxxx: xxxx: xxxx: xxxx: xxxx: xxxx

0: 15: : l: 12: 1213


This means that the original address is

0000: 0015: 0000: 0000: 0000: 0001: 0012: 1213


4. Address Space


IPv6 has a much larger address space; 2128 addresses are available. The designers of IPv6 divided the address into several categories. A few leftmost bits, called the type prefix, in each address define its category. The type prefix is variable in length, but it is designed such that no code is identical to the first part of any other code. In this way, there is no ambiguity; when an address is given, the type prefix can easily be determined.


5. Unicast Addresses

A unicast address defines a single computer. The packet sent to a unicast address must be delivered to that specific computer. IPv6 defines two types of unicast addresses: geographically based and provider-based. We discuss the second type here; the first type is left for future definition. The provider-based address is generally used by a normal host as a unicast address.

Fields for the provider-based address are as follows:


·           Type identifier: This 3-bit field defines the address as a provider-based address.


·           Registry identifier. This 5-bit field indicates the agency that has registered the address.Currently three registry centers have been defined. INTERNIC (code 11000) is the center for North America; RIPNIC (code 01000) is the center for European registration; and APNIC (code 10100) is for Asian and Pacific countries.

·           Provider identifier. This variable-length field identifies the provider for Internet access(such as an ISP). A 16-bit length is recommended for this field.


·           Subscriber identifier. When an organization subscribes to the Internet through a provider, itis assigned subscriber identification. A 24-bit length is recommended for this field.


·           Subnet identifier. Each subscriber can have many different subnetworks, and eachsubnetwork can have an identifier. The subnet identifier defines a specific subnetwork under the territory of the subscriber. A 32-bit length is recommended for this field.


·           Node identifier. The last field defines the identity of the node connected to a subnet. Alength of 48 bits is recommended for this field to make it compatible with the 48-bit link (physical) address used by Ethernet. In the future, this link address will probably be the same as the node physical address.


6. Multicast Addresses


Multicast addresses are used to define a group of hosts instead of just one. A packet sent to a multicast address must be delivered to each member of the group.

The second field is a flag that defines the group address as either permanent or transient. A permanent group address is defined by the Internet authorities and can be accessed at all times. A transient group address, on the other hand, is used only temporarily. Systems engaged in a teleconference, for example, can use a transient group address. The third field defines the scope of the group address. Many different scopes have been defined.


7. All cast Addresses


IPv6 also defines anycast addresses. An anycast address, like a multicast address, also defines a group of nodes. However, a packet destined for an anycast address is delivered to only one of the members of the anycast group, the nearest one (the one with the shortest route). Although the definition of an anycast address is still debatable, one possible use is to assign an anycast address to all routers of an ISP that covers a large logical area in the Internet.


8. Reserved Addresses


Another category in the address space is the reserved address. These addresses start with eight Os (type prefix is 00000000). A few subcategories are defined in this categor.


An unspecified address is used when a host does not know its own address and sends an inquiry to find its address. A loopback address is used by a host to test itself without going into the network. A compatible address is used during the transition from IPv4 to IPv6.


9. Local Addresses


These addresses are used when an organization wants to use IPv6 protocol without being connected to the global Internet. In other words, they provide addressing for private networks. Nobody outside the organization can send a message to the nodes using these addresses. Two types of addresses are defined for this purpose.


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