(libc.info.gz) Abstract Host Addresses

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(libc.info.gz) Host Addresses

(libc.info.gz) Host Address Data Type

 
 16.6.2.1 Internet Host Addresses
 ................................
 
 Each computer on the Internet has one or more Internet addresses,
 numbers which identify that computer among all those on the Internet.
 
    An IPv4 Internet host address is a number containing four bytes of
 data.  Historically these are divided into two parts, a "network number"
 and a "local network address number" within that network.  In the
 mid-1990s classless addresses were introduced which changed this
 behavior.  Since some functions implicitly expect the old definitions,
 we first describe the class-based network and will then describe
 classless addresses.  IPv6 uses only classless addresses and therefore
 the following paragraphs don't apply.
 
    The class-based IPv4 network number consists of the first one, two or
 three bytes; the rest of the bytes are the local address.
 
    IPv4 network numbers are registered with the Network Information
 Center (NIC), and are divided into three classes--A, B and C. The local
 network address numbers of individual machines are registered with the
 administrator of the particular network.
 
    Class A networks have single-byte numbers in the range 0 to 127.
 There are only a small number of Class A networks, but they can each
 support a very large number of hosts.  Medium-sized Class B networks
 have two-byte network numbers, with the first byte in the range 128 to
 191.  Class C networks are the smallest; they have three-byte network
 numbers, with the first byte in the range 192-255.  Thus, the first 1,
 2, or 3 bytes of an Internet address specify a network.  The remaining
 bytes of the Internet address specify the address within that network.
 
    The Class A network 0 is reserved for broadcast to all networks.  In
 addition, the host number 0 within each network is reserved for
 broadcast to all hosts in that network.  These uses are obsolete now but
 for compatibility reasons you shouldn't use network 0 and host number 0.
 
    The Class A network 127 is reserved for loopback; you can always use
 the Internet address '127.0.0.1' to refer to the host machine.
 
    Since a single machine can be a member of multiple networks, it can
 have multiple Internet host addresses.  However, there is never supposed
 to be more than one machine with the same host address.
 
    There are four forms of the "standard numbers-and-dots notation" for
 Internet addresses:
 
 'A.B.C.D'
      This specifies all four bytes of the address individually and is
      the commonly used representation.
 
 'A.B.C'
      The last part of the address, C, is interpreted as a 2-byte
      quantity.  This is useful for specifying host addresses in a Class
      B network with network address number 'A.B'.
 
 'A.B'
      The last part of the address, B, is interpreted as a 3-byte
      quantity.  This is useful for specifying host addresses in a Class
      A network with network address number A.
 
 'A'
      If only one part is given, this corresponds directly to the host
      address number.
 
    Within each part of the address, the usual C conventions for
 specifying the radix apply.  In other words, a leading '0x' or '0X'
 implies hexadecimal radix; a leading '0' implies octal; and otherwise
 decimal radix is assumed.
 
 Classless Addresses
 ...................
 
 IPv4 addresses (and IPv6 addresses also) are now considered classless;
 the distinction between classes A, B and C can be ignored.  Instead an
 IPv4 host address consists of a 32-bit address and a 32-bit mask.  The
 mask contains set bits for the network part and cleared bits for the
 host part.  The network part is contiguous from the left, with the
 remaining bits representing the host.  As a consequence, the netmask can
 simply be specified as the number of set bits.  Classes A, B and C are
 just special cases of this general rule.  For example, class A addresses
 have a netmask of '255.0.0.0' or a prefix length of 8.
 
    Classless IPv4 network addresses are written in numbers-and-dots
 notation with the prefix length appended and a slash as separator.  For
 example the class A network 10 is written as '10.0.0.0/8'.
 
 IPv6 Addresses
 ..............
 
 IPv6 addresses contain 128 bits (IPv4 has 32 bits) of data.  A host
 address is usually written as eight 16-bit hexadecimal numbers that are
 separated by colons.  Two colons are used to abbreviate strings of
 consecutive zeros.  For example, the IPv6 loopback address
 '0:0:0:0:0:0:0:1' can just be written as '::1'.
 

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(libc.info.gz) Host Addresses

(libc.info.gz) Host Address Data Type