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The IPv4 header is variable in size due to the optional 14th field (Options). The IHL field contains the size of the IPv4 header; it has 4 bits that specify the number of 32-bit words in the header. The minimum value for this field is 5, [33] which indicates a length of 5 × 32 bits = 160 bits = 20 bytes. As a 4-bit field, the maximum value is ...
"IANA IPv4 Address Space Registry". Internet Assigned Numbers Authority (IANA). 21 December 2017 The authoritative up-to-date list of IANA assignments. "ARIN Whois Database". American Registry for Internet Numbers (ARIN) Historical IP address lists: "INTERNET PROTOCOL ADDRESS SPACE".
Class Leading bits Size of network number bit field Size of rest bit field Number of networks Addresses per network Total addresses in class Start address End address Default subnet mask in dot-decimal notation CIDR notation; Class A 0 8 24 128 (2 7) 16,777,216 (2 24) 2,147,483,648 (2 31) 0.0.0.0: 127.255.255.255 [a] 255.0.0.0 / 8: Class B 10 ...
An IPv4 address has a size of 32 bits, which limits the address space to 4 294 967 296 (2 32) addresses. Of this number, some addresses are reserved for special purposes such as private networks (≈18 million addresses) and multicast addressing (≈270 million addresses).
At the top level, the IP address pool is managed by the Internet Assigned Numbers Authority (IANA). The total IPv4 address pool contains 4 294 967 296 (2 32 ) addresses, while the size of the IPv6 address pool is 2 128 ( 340 282 366 920 938 463 463 374 607 431 768 211 456 ) addresses.
Each IP address consists of a network prefix followed by a host identifier. In the classful network architecture of IPv4, the three most significant bits of the 32-bit IP address defined the size of the network prefix for unicast networking, and determined the network class A, B, or C. [3]
IPv4 reserves the entire class A address block 127.0.0.0 / 8 for use as private loopback addresses. IPv6 reserves the single address ::1 . Some are advocating reducing 127.0.0.0 / 8 to 127.0.0.0 / 16 .
Used for link-local addresses [5] between two hosts on a single link when no IP address is otherwise specified, such as would have normally been retrieved from a DHCP server 172.16.0.0/12 172.16.0.0–172.31.255.255