Internet Protocol Address

IP addresses are used to identify a particular network card somewhere on the global [Internet].  They come in two forms
* an [IPv4] address looks like ''nnn.nnn.nnn.nnn'' (ie. 192.168.1.42), and is actually a single 32 bit number.
* an [IPv6] 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 ''hhhh:hhhh:hhhh:hhhh:hhhh:hhhh:nnn.nnn.nnn.nnn''.
** 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 connection

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 192.168.0.0 and the machines could be 192.168.0.1, 192.168.0.2 and 192.168.0.254.  Notice that the first part of all the machine addresses is the same, and also the same as the network address.

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:
* 192.168.0.0/24 which means that only the first 24 bits identify the network (and so the remaining 8 identify the host)
* 192.168.0.0/255.255.255.0 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.