The IP protocol is a connectionless, unreliable protocol. TCP uses IP to establish sessions with remote computers and provides the reliability of the data transactions. IP, however, provides the hierarchical address space used by IPv4. Yet this address space is limited to fields in the IP datagram that are only 32 bits in length. When first created, it seemed like this address space would provide enough IP addresses to last for decades or more. After all, only government, educational facilities, and a few other institutions used what was then the ARPANET (the predecessor of the Internet). The address classes' original part of the IPv4 address space has pretty much been displaced by CIDR, to prevent wasting large ranges of addresses allocated to a single entity (such as class A networks).
IPv6 increases this 32-bit address space to 128 bits. At first glance, 32 bits versus 128 bits doesn't seem to be a big difference. When you consider the number of possible addresses that each of these bit ranges can provide, however, there is a tremendous difference. Fill a 32-bit field with all ones and you end up with a number just over 4 billion. A 128-bit field can provide a much larger number of possible addresses. The actual number of addresses, of course, depends on which bits are used to identify a network and which are used to identify a host on a network.
The address space that IPv4 enables can give us enough addresses to satisfy the demand today, especially when using NAT for LANs and using CIDR to reclaim wasted address space that was created by the original address classes. Yet the world of electronics today has changed the playing field. It's not just computers that need an IP address. Handheld devices, mobile phones, and other consumerdevices will likely require an IP address in the near future. NAT might work well in a LAN or a small enterprise network, but when you consider that many wireless devices will roam from one provider to another, an assigned IP address becomes more important. NAT is performed at a local level, not a national or global one.
Expanding the IP address space is not the only feature that IPv6 gives to the Internet and your LAN or WAN. Other important features include the following:
- A simpler header format for the IP datagram, which makes it possible to create faster routing techniques implemented in hardware designs.
- Support for new extensions to the IP header, as well as a means to include future expansion for additional headers that may be created later.
- The replacement of certain options left over from the IPv4 specification, as well as new options, and, again, room for expansion of additional options as required in the future.
- The capability to specify which datagrams require special handling when it comes to flow control. This capability can enable real-time handling of a stream of IP datagrams (needed, for example, for real-time voice or video communication over an IP network), a feature usually accomplished by other protocols tunneling IP.
- Authentication and encryption capabilities to provide for a secure connection.
- As you can see, there are many differences between the capabilities of IPv4 and those of IPv6.
The IPv6 Headers
Headers are used by protocols to provide information about source and destination addresses, protocols, or the payload encapsulated by the datagram. It is typical that one protocol's packet is sent as the payload of another protocol. For example, the IP datagram is usually sent across most LANs encapsulated in an Ethernet frame. At the destination, the Ethernet portion of the frame is stripped off and the IP packet information is
revealed. The IP information is then removed by the protocol stack, and the TCP (or other protocol) information is then removed before the actual data is reassembled and sent to an application.
A few of the IPv4 fields were never put to any practical use. And some of those fields no longer existin the IPv6 header.
The fields for IPv6 are as listed here:
- Version—This 4-bit field is the version of the Internet Protocol. The value for IPv4 was 4. The version for IPv6 in this field is 6. Routers and other devices use this value to determine what type of datagram is being processed.
- Traffic Class—Similar to the Flow Label field, this field enables nodes to specify a particular "traffic class," the definition of which is still being defined by many RFC documents. This field should be supported by any intervening device (such as a router) that understands this field (as it exists today), and ignored by those that do not.
- Flow Label—This 20-bit field is used to request that devices that stand between the source and the destination give special preference to this datagram. This can be compared to the Quality of Service field of IPv4 headers
- Payload Length—This 16-bit field is used to indicate the length of the payload section of the datagram. This does not include the original header of the IP datagram, but it does include any additional headers, as well as the actual payload of the datagram.
- Next Header—One of the most useful features that IPv6 provides is that additional headers can be included in a datagram, in addition to the main IPv6 header itself. There are many typ of headers, and they can be useful depending on the type of payload, or the treatment of the current datagram (such as routing techniques).
- Hop Limit—This 8-bit field is similar to a Time to Live (TTL) field used by many other pro cols. It is decremented by 1 at each router the datagram passes through. When the value reaches 0, the datagram can be discarded.
- Source Address—A 128-bit address used to describe the IP source of the datagram.
- Destination Address—A 128-bit address used to describe the IP destination address of the datagram.
This section describes just the initial IPv6 header format. In the next section you will learn about ho IPv6 can include additional headers that extend the traditional header to provide information about additional services for the IP protocol.