On 27 Jan 2006 at 10:43, Chilangisha B Changwe wrote:

> I am new to networking and I have a question on IP addressing. I know
> there are 3 classes of IP addresses; A, B, and C. My question is who
> assigns these addresses so that every computer has a unique address?
>
> Chilangisha Changwe

  Classes A, B, C (there are also D and E...) are essentially obsolete.  You
only need to know about them if you plan to make a career of networking.

  A routable network address consists of two parts: a network part and a
host (within the network) part.  Novell's IPX used 32 bits for the network
part and 16 for the host part; AppleTalk used 16 bits for the network part
and 32 for the host part.  Or perhaps I have those backwards.
  IPv4 uses 32 bits for the whole address, and splits them *variably*
between the network and host.  [This has two important advantages:  the
address fits in a single 32-bit word, and networks can be hierarchically
subnetted which means that even very central routers can keep their maps of
the Internet to a reasonable size!]

  All modern network devices that use IPv4 specify the division between
network and host parts via a variable mask, either directly as a 32-bit
pattern (written in the same "dotted quad" format as the address itself), or
simply as a count of the bits which form the network "prefix".  So the RFC
1918 "Class C" private network that contains 192.168.1.1 uses a mask of
255.255.255.0, or can be written as 192.168.1.0/24 (this is the network, not
the individual host ".1").

  The reason that career networkers need to know about classes is that there
are still pieces of network equipment and protocols in use that date back to
before the invention of masks for this purpose.  The "Class" of an address
specifies the number of network address bits as *inferred* from the address
itself.  You need to be familiar with this system to understand what these
ancient artifacts are doing, and why.

  There are four major delegators of public network address blocks.  Here in
North America, it's ARIN (American Registry of Internet Numbers).  If you
want to see who has been assigned a specific block, visit
http://www.arin.net and chose "WHOIS".
  If the block is delegated by one of the other registries, the answer will
direct you there:  RIPE in Europe, APNIC in Asia, and there's a newer one
for Latin and South America whose acronym escapes me at the moment; they all
provide "WHOIS" services for the blocks they administer.
  They typically allocate large blocks to major ISPs, who use some of those
blocks themselves and delegate others to some of their customers.

David Gillett
Sr. Network Engineer
CCNP CCSE MCSE

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