IPv4 Network Addresses in Software

Generation Code 39 in Software IPv4 Network Addresses

The IP address is divided into two parts: one part identifies the network, and the other part identifies a particular host The network address identifies the network of which a particular interface on a host is a part Two methods exist for implementing the network and host parts of an IP address: the original class-based IP addressing and the current Classless Interdomain Routing (CIDR) addressing Class-based IP addressing designates officially predetermined parts of the address for the network and host addresses, whereas CIDR addressing allows the parts to be determined dynamically using a netmask

Originally, IP addresses were organized according to classes On the Internet, networks are organized into three classes depending on their size classes A, B, and C A class A network uses only the first segment for the network address and the remaining three for the host, allowing a great many computers to be connected to the same network Most IP addresses reference smaller, class C, networks For a class C network, the first three segments are used to identify the network, and only the last segment identifies the host Altogether, this forms a unique address with which to identify any network interface on computers in a TCP/IP network For example, in the IP address 192168172, the network part is 1921681 and the interface/host part is 72 The interface/host is a part of a network whose own address is 19216810 In a class C network, the first three numbers identify the network part of the IP address This part is divided into three network numbers, each identifying a subnet Networks on the Internet are organized into subnets, beginning with the largest and narrowing to small subnetworks The last number is used to identify a particular computer, referred to as a host You can think of the Internet as a series of networks with subnetworks; these subnetworks have their own subnetworks The rightmost number identifies the host computer, and the

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number preceding it identifies the subnetwork of which the computer is a part The number to the left of that identifies the network the subnetwork is part of, and so on The Internet address 1921681874 references the fourth computer connected to the network identified by the number 187 Network 187 is a subnet to a larger network identified as 168 This larger network is itself a subnet of the network identified as 192 Here s how it breaks down:

Systems derive the network address from the host address using the netmask You can think of an IP address as a series of 32 binary bits, some of which are used for the network and the remainder for the host The netmask has the network set of bits set to 1s, with the host bits set to 0s (see Figure 34-1) In a netmask for a standard class-based IP address, all the numbers in the network part of the host address are set to 255, and the host part is set to 0 This has the effect of setting all the binary bits making up the network address in the netmask to 1s This, then, is your netmask So, the netmask for the Class C host address 192168172 is 2552552550 The network part, 1921681, has been set to 255255255, and the host part, 72, has been set to 0 Systems can then use your netmask to derive your network address from your host address They can determine what part of your host address makes up your network address and what those numbers are For those familiar with computer programming, a bitwise AND operation on the netmask and the host address results in zeroing the host part, leaving you with the network part of the host address You can think of the address as being implemented as a four-byte integer, with each byte corresponding to a segment of the address In a class C address, the three network segments correspond to the first three bytes and the host segment corresponds to the fourth byte A netmask is designed to mask out the host part of the address, leaving the network segments alone In the netmask for a standard class C network, the first three bytes are all 1s and the last byte consists of 0s The 0s in the last byte mask out the host part of the address, and the 1s in the first three bytes leave the network part of the address alone Figure 34-1 shows the bitwise operation of the netmask on the address 19216814 When it is applied to the address 19216814, the network address (1921681) remains and the host address (4) is masked out, giving you 19216810 as the network address The netmask as used in Classless Interdomain Routing (CIDR) is much more flexible Instead of having the size of the network address and its mask determined by the network class, it is determined by a number attached to the end of the IP address This number simply specifies the size of the network address, how many bits in the address it takes up For example, in an IP address whose network part takes up the first three bytes (segments), the number of bits used for that network part is 24 eight bits to a byte (segment) Instead of using a netmask to determine the network address, the number for the network size is attached to the end of the address with a slash, as shown here:

192168172/24

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