Data Communications Networking Devices - 4th Ed.pdf

2.5 THE INTERNET _______________________________________________________________ 221bit subnet mask must be created for each network, with bits set to `1' in the subnetmask to indicate the network portion of the IP address, while bits are set to `0' toindicate the host identi®er portion. Thus, the Class B subnet address formatillustrated in the lower portion of Figure 2.30 would require the following 32-bitsubnet mask:11111111 11111111 00000000 00000000The prior mask would then be entered as 255.255.0.0 in dotted decimal representationinto a router con®guration screen as well as in software con®gurationscreens on TCP/IP program stacks operating on each subnet. Concerning thelatter, you must then con®gure each station to indicate its subnet and host identi-®er so that each station obtains a full four-digit dotted decimal address.Although the above example used octet boundaries for creating the subnet mask,this is not an addressing requirement. For example, you could assign the followingmask to a network:11111111 11111111 00001110 00001100The only submask restriction is to assign 1's to at least all the network identi®erpositions, resulting in the ability to extend masking into the host identi®er ®eld ifyou desire to arrange the speci®c assignment of addresses to computers. However,doing so can make it more dif®cult to verify the correct assignment of addresses inrouters and workstations. For this reason it is highly recommended that youshould implement subnet masking on integral octet boundaries.Domain Name ServiceAddressing on a TCP/IP network occurs through the use of four decimal numbersranging from 0 to 255, which are separated from one another by a dot. This dotteddecimal notation represents a 32-bit address which consists of an assigned networknumber and a host number as previously described during our examination of IPaddressing. Since numeric addresses are dif®cult to work with, TCP/IP alsosupports a naming convention based upon English words or mnemonics that areeasier both to work with and to remember. The translation of English words ormnemonics to 32-bit IP addresses is performed by a Domain Name Server. Eachnetwork normally has at least one Domain Name Server, and communicationsestablished between such servers on TCP/IP networks connected to the Internetare referred to as a Domain Name Service DNS).The Domain Name Service is the naming protocol used in the TCP/IP protocolsuite which enables IP routing to occur indirectly through the use of names insteadof IP addresses. To accomplish this, DNS provides a domain name to IP addresstranslation service.A domain is a subdivision of a wide area network. When applied to the Internetthe collection of networks interconnected to one another), there are six top-leveland seven pending domain names which were speci®ed by the Internet Network