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Netmask
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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 39-1). In a standard class-based IP address, all the numbers in the network part of your 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 to 1s. This, then, is your netmask. So, the netmask for the host address 192.168.1.72 is 255.255.255.0. The network part, 192.168.1, has been set to 255.255.255, 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.
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Figure 39-1: Class-based netmask operations 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 39-1 shows the bitwise operation of the netmask on the address 192.168.1.4. This is a class C address to the mask, which consists of twenty-four 1s making up the first three bytes and eight
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0s making up the last byte. When it is applied to the address 192.168.1.4, the network address remains (192.168.1) and the host address is masked out (4), giving you 192.168.1.0 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:
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192.168.1.72/24
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CIDR gives you the advantage of specifying networks that are any size bits, instead of only three possible segments. You could have a network whose addresses take up 14 bits, 22 bits, or even 25 bits. The host address can use whatever bits are left over. An IP address with 21 bits for the network can cover host addresses using the remaining 11 bits, 0 to 2,047.
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Classless Interdomain Routing (CIDR)
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Currently, the class-based organization of IP addresses is being replaced by the CIDR format. CIDR was designed for midsized networks, those between a class C and classes with numbers of hosts greater than 256 and smaller than 65,534. A class C network based IP address using only one segment for hosts uses only one segment, an 8-bit integer, with a maximum value of 256. A class B network based IP address uses two segments, which make up a 16-bit integer whose maximum value is 65,534. You can think of an address as a 32-bit integer taking up four bytes, where each byte is eight bits. Each segment conforms to one of the four bytes. A class C network uses three segments, or 24 bits, to make up its network address. A class B network, in turn, uses two segments, or 16 bits, for its address. With this scheme, allowable host and network addresses are changed an entire byte at a time, segment to segment. With CIDR addressing, you can define host and network addresses by bits, instead of whole segments. For example, you can use CIDR addressing to expand the host segment from eight bits to nine, rather than having to jump it to a class B 16 bits (two segments). CIDR addressing notation achieves this by incorporating netmask information in the IP address (the netmask is applied to an IP address to determine the network part of the address). In the CIDR notation, the number of bits making up the network address is placed after the IP address, following a slash. For example, the CIDR form of the class C 192.168.187.4 IP address is
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192.168.187.4/24
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Figure 39-2 shows an example of a CIDR address and its network mask. The IP address is 192.168.1.6 with a network mask of 22 bits, 192.168.1.6/22. The network address takes up the first 22 bits of the IP address, and the remaining 10 bits are used for the host address. The host address is taking up the equivalent of a class-based IP address's fourth segment (8 bits) and two bits from the third segment.
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Figure 39-2: CIDR addressing Table 39-3 lists the different CIDR network masks available along with the maximum number of hosts. Both the short form and the full forms of the netmask are listed. Table 39-3: CIDR Network Masks Full Form Maximum Number of Hosts /255.0.0.0 /255.255.0.0 /255.255.128.0 /255.255.192.0 /255.255.224.0 /255.255.240.0 /255.255.248.0 /255.255.252.0 /255.255.254.0 /255.255.255.0 /255.255.255.128 /255.255.255.192 /255.255.255.224 /255.255.255.240 /255.255.255.248 /255.255.255.252 16,777,215 (A class) 65,535 (B class) 32,767 16,383 8,191 4,095 2,047 1,023 511 255 (C class) 127 63 31 15 7 3
Short Form /8 /16 /17 /18 /19 /20 /21 /22 /23 /24 /25 /26 /27 /28 /29 /30
The network address for any standard class C IP address takes up the first three segments, 24 bits. If you want to create a network with a maximum of 512 hosts, you can give them IP addresses where the network address is 23 bits and the host address takes up 9 bits (0 511). The IP address notation remains the same, however, using the four 8-bit segments. This means a given segment's number could be used for both a network address and a host address. Segments are no longer wholly part of either the host address or the network address. Assigning a 23-bit network address and a 9-bit host address means that the number in the third segment is part of both the network address and the host address, the first seven bits for the network and the last bit for the host. In this following example, the third number, 145, is used as the end of the network address and as the beginning of the host address:
192.168.145.67/23
This situation complicates CIDR addressing, and in some cases the only way to represent the address is to specify two or more network addresses. Check RFC 1520 at www.ietf.org for more details. Note A simple way to calculate the number of hosts a network can address is to take the number of bits in its host segment as a power of 2, then subtract 2 that is, 2 to the number of host bits, minus 2. For example, an 8-bit host segment would be 2 to the power of 8, which equals 256. Subtract 2, one for the broadcast address, 255, and one for the zero value, 000, to leave you with 254 possible hosts. CIDR also allows a network administrator to take what is officially the host part of an IP address and break it up into subnetworks with fewer hosts. This is referred to as subnetting. A given network will have its official IP network address recognized on the Internet or by a larger network. The network administrator for that network could, in turn, create several smaller networks within it using CIDR network masking. A classic example is to take a standard class C network with 254 hosts and break it up into two smaller networks, each with 64 hosts. You do this by using a CIDR netmask to take a bit from the host part of the IP address and use it for the subnetworks. Numbers within the range of the original 254 addresses whose first bit would be set to 1 would represent one subnet, and the others, whose first bit would be set to 0, would constitute the remaining network. In the network whose network address is 192.168.187.0, where the last segment is used for the hostnames, that last host segment could be further split into two subnets, each with its own hosts. For two subnets, you would use the first bit in the last 8-bit segment for the network. The remaining seven bits could then be used for host addresses, giving you a range of 127 hosts per network. The subnet whose bit is set to 0 would have a range of 1 to 127, with a CIDR netmask of 25. The 8-bit segment for the first host would be 00000001. So, the host with the address of 1 in that network would have this IP address:
192.168.187.1/25
For the subnet where the first bit is 1, the first host would have an address of 129, with the CIDR netmask of 25, as shown here. The 8-bit sequence for the first host would be 10000001.
192.168.187.129/25
Each subnet would have a set of 126 addresses, the first from 1 to 126, and the second from 129 to 254; 127 is the broadcast address for the first subnet, and 128 is the network address for the second subnet. The possible subnets and their masks that you could use are shown here. Subnetwork First subnet network address Second subnet network address First subnet broadcast address Second subnet broadcast address First address in first subnet First address in second subnet Last address in first subnet CIDR Address .0/25 .128/25 .127/25 .255/25 .1/25 .129/25 .126/25 Binary Mask 00000000 10000000 01111111 11111111 00000001 10000001 01111110
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