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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.
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