Information Technology Reference
In-Depth Information
New and Improved IPv6 Concepts
Several elements of the IPv4 protocol could use some enhancements. Fortunately, IPv6
incorporates those enhancements as well as new features directly into the protocol
specification to provide better and additional functionality.
The following list includes new concepts and new implementations of old concepts in IPv6:
■
Larger address space (128-bit vs. 32-bit).
■
Autoconfiguration of Internet-accessible addresses with or without DHCP. (Without
DHCP, it's called
stateless autoconfiguration
.)
■
More efficient IP header (fewer fields and no checksum).
■
Fixed-length IP header (the IPv4 header is variable length) with extension headers
beyond the standard fixed length to provide enhancements.
■
Built-in IP mobility and security. (Although available in IPv4, the IPv6 implementation
is a much better implementation.)
■
Built-in transition schemes to allow integration of the IPv4 and IPv6 spaces.
■
ARP broadcast messages replaced with multicast request.
Here are more details about these features:
128-Bit Address Space
The new 128-bit address space will provide unique addresses
for the foreseeable future. Although I would like to say that we will never use up all of
the addresses, history may prove me wrong. The number of unique addresses in the IPv6
space is 2
128
, or 3.4
×
10
38
, addresses. How big is that number? It's enough for toasters and
refrigerators (and maybe even cars) to all have their own addresses.
As a point of reference, the nearest black hole to Earth is 1,600 light years away. If you
were to stack 4mm BB pellets from here to the nearest black hole and back, you would need
1.51
×
10
22
BBs. This means you could uniquely address each BB from Earth to the black
hole and back and still have quite a few addresses left over.
Another way to look at it is that the IPv6 address space is big enough to provide more than
1 million addresses per square inch of the surface area of the earth (oceans included).
Autoconfiguration and Stateless Autoconfiguration
Autoconfiguration is another
added/improved feature of IPv6. We've used DHCP for a while to assign IP addresses to
client machines. You should even remember that APIPA can be used to assign addresses
automatically to Microsoft DHCP client machines in the absence of a DHCP server. The
problem with APIPA is that it confines communication between machines to a local LAN
(no default gateway). What if a client machine could ask whether there was a router on the
LAN and what network it was on? If the client machine knew that, it could not only assign
itself an address, it could also choose the appropriate network and default gateway. The
stateless autoconfiguration functionality of IPv6 allows the clients to do this.
Improved IPv6 Header
The IPv6 header is more efficient than the IPv4 header because it
is fixed length (with extensions possible) and has only a few fields. The IPv6 header consists
of a total of 40 bytes:
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