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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Addressing
                          9  IP "Classful" (Conventional) Addressing

Previous Topic/Section
IP Address Class A, B and C Network and Host Capacities
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Pages in Current Topic/Section
1
2
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IP Reserved, Loopback and Private Addresses
Next Topic/Section

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.



Previous Topic/Section
IP Address Class A, B and C Network and Host Capacities
Previous Page
Pages in Current Topic/Section
1
2
Next Page
IP Reserved, Loopback and Private Addresses
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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