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IP Addresses With Special Meanings
(Page 1 of 2)
Most IP addresses have the usual
meaning I have described in the preceding topics in this section: they
refer to an interface to a device on a TCP/IP network. However, some
IP addresses do not refer directly to specific hardware devices in this
manner. Instead, they are used to refer indirectly to one
or more devices. To draw an analogy with language, most IP addresses
refer to proper nouns, like John or the red table
in the corner. However, some are used more the way we use pronouns
such as this one or that group over there. I
call these IP addresses with special meanings.
Special Network ID and Host ID Address Patterns
Special IP addresses are constructed
by replacing the normal network ID or host ID (or both) in an IP address
with one of two special patterns. The two patterns are:
- All Zeroes: When the network ID or host
ID bits are replaced by a set of all zeroes, the special meaning is
the equivalent of the pronoun this, referring
to whatever was replaced. It can also be interpreted as the default
or the current. So for example, if we replace the network
ID with all zeroes but leave the host ID alone, the resulting address
means the device with the host ID given, on this network.
Or alternatively, the device with the host ID specified, on the
default network or the current network.
- All Ones: When the network ID or host
ID bits are replaced by a set of all ones, this has the special meaning
of all. So replacing the host ID with all
ones means the IP address refers to all hosts on the network. This is
generally used as a broadcast address for sending a message to everyone.
Key Concept: When the network ID or host ID of an IP address is replaced by a pattern of all ones or all zeroes, the result is an address with a special meaning. Examples of such addresses include all hosts broadcast addresses, and addresses that refer to a specific host or a whole network.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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