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The routing prefix of an address is identified by the subnet mask, written in the same form used for IP addresses. For example, the subnet mask for a routing prefix that is composed of the most-significant 24 bits of an IPv4 address is written as 255.255.255.0.
The Internet Engineering Task Force (IETF) has reserved the IPv4 address block 169.254.0.0 / 16 (169.254.0.0 – 169.254.255.255) for link-local addressing. [1] The entire range may be used for this purpose, except for the first 256 and last 256 addresses (169.254.0.0 / 24 and 169.254.255.0 / 24), which are reserved for future use and must not be selected by a host using this dynamic ...
This means that 256 /8 address blocks fit into the entire IPv4 space. As IPv4 address exhaustion has advanced to its final stages, some organizations, such as Stanford University, formerly using 36.0.0.0 / 8, have returned their allocated blocks (in this case to APNIC) to assist in the delay of the exhaustion date.
A subnet mask is a bitmask that encodes the prefix length associated with an IPv4 address or network in quad-dotted notation: 32 bits, starting with a number of 1-bits equal to the prefix length, ending with 0-bits, and encoded in four-part dotted-decimal format: 255.255.255.0. A subnet mask encodes the same information as a prefix length but ...
Both IP versions however use the CIDR concept and notation. In this, the IP address is followed by a slash and the number (in decimal) of bits used for the network part, also called the routing prefix. For example, an IPv4 address and its subnet mask may be 192.0.2.1 and 255.255.255.0, respectively.
However, this does not mean that every address ending in 0 or 255 cannot be used as a host address. For example, in the / 16 subnet 192.168.0.0 / 255.255.0.0, which is equivalent to the address range 192.168.0.0 – 192.168.255.255, the broadcast address is 192.168.255.255.
Special address blocks Address block Address range Number of addresses Scope Description 0.0.0.0/8 0.0.0.0–0.255.255.255 16 777 216: Software Current (local, "this") network [1] 10.0.0.0/8 10.0.0.0–10.255.255.255 16 777 216: Private network Used for local communications within a private network [3] 100.64.0.0/10 100.64.0.0–100.127.255.255 ...
IP addresses in dot-decimal notation are also presented in CIDR notation, in which the IP address is suffixed with a slash and a number, used to specify the length of the associated routing prefix. For example, 127.0.0.1/8 specifies that the IP address has an eight-bit routing prefix, and therefore the subnet mask 255.0.0.0 .