The TCP/IP Guide - IP Addresses With Special Meanings

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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.

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.