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| <h5 class="subsubsection">16.6.2.1 Internet Host Addresses</h5> |
| |
| <p><a name="index-host-address_002c-Internet-1687"></a><a name="index-Internet-host-address-1688"></a> |
| |
| <p><a name="index-network-number-1689"></a><a name="index-local-network-address-number-1690"></a>An IPv4 Internet host address is a number containing four bytes of data. |
| Historically these are divided into two parts, a <dfn>network number</dfn> and a |
| <dfn>local network address number</dfn> 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. |
| |
| <p>The class-based IPv4 network number consists of the first one, two or |
| three bytes; the rest of the bytes are the local address. |
| |
| <p>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. |
| |
| <p>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. |
| |
| <p>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. |
| |
| <p>The Class A network 127 is reserved for loopback; you can always use |
| the Internet address ‘<samp><span class="samp">127.0.0.1</span></samp>’ to refer to the host machine. |
| |
| <p>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. |
| |
| <!-- !!! this section could document the IN_CLASS* macros in <netinet/in.h>. --> |
| <!-- No, it shouldn't since they're obsolete. --> |
| <p><a name="index-standard-dot-notation_002c-for-Internet-addresses-1691"></a><a name="index-dot-notation_002c-for-Internet-addresses-1692"></a>There are four forms of the <dfn>standard numbers-and-dots notation</dfn> |
| for Internet addresses: |
| |
| <dl> |
| <dt><var>a</var><code>.</code><var>b</var><code>.</code><var>c</var><code>.</code><var>d</var><dd>This specifies all four bytes of the address individually and is the |
| commonly used representation. |
| |
| <br><dt><var>a</var><code>.</code><var>b</var><code>.</code><var>c</var><dd>The last part of the address, <var>c</var>, is interpreted as a 2-byte quantity. |
| This is useful for specifying host addresses in a Class B network with |
| network address number <var>a</var><code>.</code><var>b</var>. |
| |
| <br><dt><var>a</var><code>.</code><var>b</var><dd>The last part of the address, <var>b</var>, is interpreted as a 3-byte quantity. |
| This is useful for specifying host addresses in a Class A network with |
| network address number <var>a</var>. |
| |
| <br><dt><var>a</var><dd>If only one part is given, this corresponds directly to the host address |
| number. |
| </dl> |
| |
| <p>Within each part of the address, the usual C conventions for specifying |
| the radix apply. In other words, a leading ‘<samp><span class="samp">0x</span></samp>’ or ‘<samp><span class="samp">0X</span></samp>’ implies |
| hexadecimal radix; a leading ‘<samp><span class="samp">0</span></samp>’ implies octal; and otherwise decimal |
| radix is assumed. |
| |
| <h5 class="subsubheading">Classless Addresses</h5> |
| |
| <p>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 ‘<samp><span class="samp">255.0.0.0</span></samp>’ or a prefix length of 8. |
| |
| <p>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 ‘<samp><span class="samp">10.0.0.0/8</span></samp>’. |
| |
| <h5 class="subsubheading">IPv6 Addresses</h5> |
| |
| <p>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 |
| ‘<samp><span class="samp">0:0:0:0:0:0:0:1</span></samp>’ can just be written as ‘<samp><span class="samp">::1</span></samp>’. |
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