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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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3.1.2 <strong>IP</strong> subnets<br />

Special use <strong>IP</strong> addresses<br />

RFC 3330 discusses special use <strong>IP</strong> addresses. We provide a brief description of<br />

these <strong>IP</strong> addresses in Table 3-1.<br />

Table 3-1 Special use <strong>IP</strong> addresses<br />

Address block Present use<br />

0.0.0.0/8 “This” network<br />

14.0.0.0/8 Public-data networks<br />

24.0.0.0/8 Cable television networks<br />

39.0.0.0/8 Reserved but subject to allocation<br />

128.0.0.0/16 Reserved but subject to allocation<br />

169.254.0.0/16 Link local<br />

191.255.0.0/16 Reserved but subject to allocation<br />

192.0.0.0/24 Reserved but subject to allocation<br />

192.0.2.0/24 Test-Net 192.88.99.0/24 6to4 relay anycast<br />

198.18.0.0/15 Network interconnect device benchmark testing<br />

223.255.255.0/24 Reserved but subject to allocation<br />

224.0.0.0/4 Multicast<br />

240.0.0.0/4 Reserved for future use<br />

Due to the explosive growth of the Internet, the principle of assigned <strong>IP</strong><br />

addresses became too inflexible to allow easy changes to local network<br />

configurations. Those changes might occur when:<br />

► A new type of physical network is installed at a location.<br />

► Growth of the number of hosts requires splitting the local network into two or<br />

more separate networks.<br />

► Growing distances require splitting a network into smaller networks, with<br />

gateways between them.<br />

To avoid having to request additional <strong>IP</strong> network addresses, the concept of <strong>IP</strong><br />

subnetting was introduced. The assignment of subnets is done locally. The entire<br />

network still appears as one <strong>IP</strong> network to the outside world.<br />

72 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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