There is a subtle difference between these Given a numerical IP address

There is a subtle difference between these given a

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There is a subtle difference between these. Given a numerical IP address, datagrams can find their way precisely to the correct network and machine. The textual information is not sufficient however because, while the hostname is unique, the remainder of the address (the domain name) is usually a generic name for a group of networks - and we don't know how to choose the right one. A logical domain, like the above examples, can encompass any number of different networks. For example, the domain name `uio.no' encompasses all of the subnets under the address 129.240.*.* . The IP packets need to know which subnet the machine is on in order to get to their destination, because the text name only says that they should to to 120.240.*.host . The * is unknown. To complete this information, we need a database which maps internet domain names to internet addresses. This mapping is performed by the Domain Name Service (DNS) or Berkeley Internet Name Domain (BIND) which we shall discuss below. 7.4.2 Netmask and broadcast address Each address consists of two parts: a network address and a host address . A system variable called the netmask decides how IP addresses are interpreted locally. The netmask decides the boundary between how many bits of the IP address will be kept for hosts and how many will be kept for the network location name. There is thus a trade off between the number of allowed domains and the number of hosts which can be coupled to each subnet. Subnets are usually separated by routers, so the question is how many machines do we want on one side of a router?
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The netmask only has a meaning as a binary number. When you look at the netmask, you have to ask yourself - which bits are ones and which are zeroes? The bits which are ones decide which bits can be used to specify the domain and the subnets within the domain. The bits which are zeroes decide which are hostnames on each subnet. The local network administrator decides how the netmask is to be used. Figure 7.7: The netmask sets the division between network address and host address in the 4-byte IP address. The most common situation is that the first three numbers xxx.yyy.zzz represent the domain and the last number mmm represents the machine. In this case the netmask is 255.255.255.0 , leaving the last byte for machine addresses. It is only possible to have 254 different machines in the domain with address xxx.yyy.zzz with this netmask. If we wanted more, we would have to introduce a different domain name for the extra hosts! If we wanted more machines on each subnet, we would have to change the netmask and the definitions of the domain address. By making the netmask 255.255.248.0 , as in the figure above, we add an extra bit to the host part. Thus a total of hosts could use the same domain name. One address is always reserved by the internet protocol, namely the broadcast address . This is an address which is used like a wildcard - to refer to all machines in a given domain simultaneously. Another address is reserved as an address for the network itself. Usually xxx.yyy.zzz.0
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