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Internet Protocol Address

IP addresses are used to identify a particular network card somewhere
on the global Internet [1]. They come in two forms

* an IPv4 [2] address looks like _nnn.nnn.nnn.nnn_ (ie., and is actually a single 32 bit number.
* an IPv6 [3] address is more complex. It is 128 bits long, and can
have several forms.

* _hhhh:hhhh:hhhh:hhhh:hhhh:hhhh:hhhh:hhhh_ is the full form, where
each _h_ is a hex digit from 0-9, A-F or a-f, or,
* if any of the blocks of _hhhh_ are all zeros, it can be shortened
to something like fe80::2a0:ccff:fee1:4580 where the _::_ means fill
out those positions with zeros.
* Finally, there's a third format that uses an IPv4-style address to
replace the last two blocks, so it looks like
* Example: 2001:470:1f00:223:d00f:d00f:d00f:d00f

Since an IP address identifies a network card, a machine can have
more than one address. This can happen for several reasons

* a single network adaptor can have multiple addresses. This is often
called IP Aliasing, and is normally used to host multiple web sites on
a single machine.
* you can have multiple network adaptors in one machine, like

* multiple Ethernet cards or
* an Ethernet and an 802.11b wireless card, or
* one address for your Ethernet card, and another for your PPP

IP addresses are also used to specify networks, as well as machine
interfaces. A network's address does not use the full length address
available, but only a prefix.

For example, you might have a home network, with three machines. The
network could be and the machines could be, and Notice that the first part of all the
machine addresses is the same, and also the same as the network

There's one extra thing that is normally used when describing a
network address, and that is exactly what part of the address refers
to the network (and is common to all its machines) and what part is
used to distinguish individual machines.

This is called the _netmask_, and can be represented two ways:

* which means that only the first 24 bits identify the
network (and so the remaining 8 identify the host)
* which means the same thing, but is
expressed as a bit mask with 1 bits for the network portion (if you
don't get that, don't worry)

IPv6 network addresses generally use the first format (since it's
enough hassle to type the address, let alone a mask as well), which
IPv4 network addresses can use either.

[1] http://melbournewireless.org.au/?Internet
[2] http://melbournewireless.org.au/?IPv4
[3] http://melbournewireless.org.au/?IPv6

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