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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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1
2
Next Page
IP Reserved, Loopback and Private Addresses
Next Topic/Section

IP Addresses With Special Meanings
(Page 2 of 2)

Specific IP Address Patterns With Special Meanings

Since there are many network IDs and host IDs, there are also many of these “special” addresses. A small number are universal across the entire TCP/IP network, while others exist for each network ID or host ID. Since there are two “special patterns” that can be applied to the network ID, host ID or both, this yields six potential combinations, each of which has its own special meaning. Of these, five are used.

Table 46 describes each of these special meanings. In the table I have also provided three examples, one from each of Class A, B and C. This shows how an IP address in each of the common classes can be modified to each of the “special meaning” forms. (The first row shows the examples in their normal form, for reference.)


Table 46: IP Address Patterns With Special Meanings

Network ID

Host ID

Class A Example

Class B Example

Class C Example

Special Meaning and Description

Network ID

Host ID

77.91.215.5

154.3.99.6

227.82.157.160

Normal Meaning: Refers to a specific device.

Network ID

All Zeroes

77.0.0.0

154.3.0.0

227.82.157.0

“The Specified Network”: This notation, with a “0” at the end of the address, refers to an entire network.

All Zeroes

Host ID

0.91.215.5

0.0.99.6

0.0.0.160

“Specified Host On This Network”: This addresses a host on the current or default network when the network ID is not known, or when it doesn't need to be explicitly stated.

All Zeroes

All Zeroes

0.0.0.0

“Me”: (Alternately, “this host”, or “the current/default host”). Used by a device to refer to itself when it doesn't know its own IP address. The most common use is when a device attempts to determine its address using a host-configuration protocol like DHCP. May also be used to indicate that any address of a multihomed host may be used.

Network ID

All Ones

77.255.255.255

154.3.255.255

227.82.157.255

“All Hosts On The Specified Network”: Used for broadcasting to all hosts on the local network.

All Ones

All Ones

255.255.255.255

“All Hosts On The Network”: Specifies a global broadcast to all hosts on the directly-connected network. Note that there is no address that would imply sending to all hosts everywhere on the global Internet, since this would be very inefficient and costly.


Note: The missing combination from Table 46 is that of the network ID being all ones and the host ID normal. Semantically, this would refer to “all hosts of a specific ID on all networks”, which doesn't really mean anything useful in practice, so it's not used. Note also that in theory, a special address where the network ID is all zeroes and the host ID is all ones would have the same meaning as the all-ones limited broadcast address. The latter is used instead, however, because it is more general, not requiring knowledge of where the division is between the network ID and host ID.


IP Address Limitations Due to Special Meaning Patterns

Since the all-zeroes and all-ones patterns are reserved for these special meanings, they cannot be used for regular IP addresses. This is why, when we looked at the number of hosts per network in each of the classes, we had to subtract two from the theoretical maximum: one for the all-zeroes case and one for the all-ones case.

Similarly, the network ID cannot be all zeroes either. However, this doesn't require specific exclusion because the entire block of addresses with “0” in the first octet (0.x.x.x) is one of the reserved sets of IP addresses. These reserved addresses, described in the next topic, further restrict the use of certain addresses in the IP address space for “regular” uses.


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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