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< Day Day Up > |
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As noted previously, the traditional IPv4 TCP/IP address is organized into four segments, consisting of numbers separated by periods. This kind of address is still in wide use and is what people commonly refer to as an IP address. Part of an IP address is used for the network address, and the other part is used to identify a particular interface on a host in that network. You should realize that IP addresses are assigned to interfaces-such as Ethernet cards or modems-and not to the host computer. Usually a computer has only one interface and is accessed using only that interface's IP address. In that regard, an IP address can be thought of as identifying a particular host system on a network, and so the IP address is usually referred to as the host address.
In fact, though, a host system could have several interfaces, each with its own IP address. This is the case for computers that operate as gateways and firewalls from the local network to the Internet. One interface usually connects to the LAN and another to the Internet, as by two Ethernet cards. Each interface (such as an Ethernet card) has its own IP address. For example, when you use the Red Hat Network Configuration tool to specify an IP address for an Ethernet card on your system, the Devices panel lists an entry for each Ethernet card installed on your computer, beginning with eth0 for the first. Opening up a Device window, you can select the TCP protocol in the Protocols panel to open a TCP/IP setting window where you can enter the card's IP address. Other Ethernet cards have their own IP addresses. Currently, the Linux kernel can support up to four network adapters. If you use a modem to connect to an ISP, you would set up a PPP interface that would also have its own IP address (usually dynamically assigned by the ISP). Remembering this distinction is important if you plan to use Linux to set up a local or home network, using Linux as your gateway machine to the Internet (see the section "IP Masquerading" in Chapter 19).
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 192.168.1.72, the network part is 192.168.1 and the interface/host part is 72. The interface/host is a part of a network whose own address is 192.168.1.0.
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 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 192.168.187.4 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:
|
192.168.187.4 |
IPv4 address |
|
192.168.187 |
Network identification |
|
4 |
Host identification |
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 38-1). 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.
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 corresponds 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 38-1 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 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:
192.168.1.72/24
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 takes 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.
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 8 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 8 bits to 9, 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:
192.168.187.4/24
Figure 38-2 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 2 bits from the third segment.
Table 38-3 lists the different IPv4 CIDR network masks available along with the maximum number of hosts. Both the short forms and the full forms of the netmasks are listed.
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Short Form |
Full Form |
Maximum Number of Hosts |
|---|---|---|
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/8 |
/255.0.0.0 |
16,777,215 (A class) |
|
/16 |
/255.255.0.0 |
65,535 (B class) |
|
/17 |
/255.255.128.0 |
32,767 |
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/18 |
/255.255.192.0 |
16,383 |
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/19 |
/255.255.224.0 |
8,191 |
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/20 |
/255.255.240.0 |
4,095 |
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/21 |
/255.255.248.0 |
2,047 |
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/22 |
/255.255.252.0 |
1,023 |
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/23 |
/255.255.254.0 |
511 |
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/24 |
/255.255.255.0 |
255 (C class) |
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/25 |
/255.255.255.128 |
127 |
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/26 |
/255.255.255.192 |
63 |
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/27 |
/255.255.255.224 |
31 |
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/28 |
/255.255.255.240 |
15 |
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/29 |
/255.255.255.248 |
7 |
|
/30 |
/255.255.255.252 |
3 |
The network address for any standard class C IPv4 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 7 bits for the network and the last bit for the host. In the 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 (1 for the broadcast address, 255, and 1 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 7 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 in Table 38-4.
|
Subnetworks |
CIDR Address |
Binary Mask |
|---|---|---|
|
First subnet network address |
.0/25 |
00000000 |
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Second subnet network address |
.128/25 |
10000000 |
|
First subnet broadcast address |
.127/25 |
01111111 |
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Second subnet broadcast address |
.255/25 |
11111111 |
|
First address in first subnet |
.1/25 |
00000001 |
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First address in second subnet |
.129/25 |
10000001 |
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Last address in first subnet |
.126/25 |
01111110 |
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Last address in second subnet |
.254/25 |
11111110 |
IPv6 CIDR addressing works much the same as with the IPv4 method. The number of bits used for the network information is indicated by number following the address. A host (interface) address could take up much more than the 64 bits it usually does in an IPv6 address, making the network prefix (address) section smaller than 64 bits. The amount of bits that the network prefix uses is indicated by the following number. In the next example the network prefix (address) uses only the first 48 bits of the IPv6 address, and the host address uses the remaining 80 bits.
FEC0:0000:0000:0000:FEDC:BA98:7654:3210/48
You can also used the compressed version with
FEC0::FEDC:BA98:7654:3210/48
Though you can use CIDR to subnet addresses, IPv6 also supports a subnet field that can be used for subnets.
IP addresses are officially allocated by IANA, which manages all aspects of Internet addressing (www.iana.org). IANA oversees Internet Registries (IRs), which, in turn, maintain Internet addresses on regional and local levels. The Internet Registry for the Americas is the American Registry for Internet Numbers (ARIN), whose Web site is at www.arin.net. These addresses are provided to users by Internet service providers (ISPs). You can obtain your own Internet address from an ISP, or if you are on a network already connected to the Internet, your network administrator can assign you one. If you are using an ISP, the ISP may temporarily assign one from a pool it has on hand with each use.
Certain numbers are reserved. The numbers 127, 0, or 255 cannot be part of an official IP address. The number 127 is used to designate the network address for the loopback interface on your system. The loopback interface enables users on your system to communicate with each other within the system without having to route through a network connection. Its network address would be 127.0.0.0 and its IP address is 127.0.0.1. For class-based IP addressing, the number 255 is a special broadcast identifier you can use to broadcast messages to all sites on a network. Using 255 for any part of the IP address references all nodes connected at that level. For example, 192.168.255.255 broadcasts a message to all computers on network 192.168, all its subnetworks, and their hosts. The address 192.168.187.255 broadcasts to every computer on the local network. If you use 0 for the network part of the address, the host number references a computer within your local network. For example, 0.0.0.6 references the sixth computer in your local network. If you want to broadcast to all computers on your local network, you can use the number 0.0.0.255. For CIDR IP addressing, the broadcast address may appear much like a normal IP address. As indicated in the preceding section, CIDR addressing allows the use of any number of bits to make up the IP address for either the network or the host part. For a broadcast address, the host part must have all its bits set to 1 (see Figure 38-3).
A special set of numbers is reserved for use on non-Internet LANs (RFC 1918). These are numbers that begin with the special network number 192.168 (for class C networks), as used in these examples. If you are setting up a LAN, such as a small business or a home network, you are free to use these numbers for your local machines. You can set up an intranet using network cards, such as Ethernet cards and Ethernet hubs, and then configure your machines with IP addresses starting from 192.168.1.1. The host segment can go up to 256. If you have three machines on your home network, you could give them the addresses 192.168.1.1, 192.168.1.2, and 192.168.1.3. You can implement Internet services, such as FTP, Web, and mail services, on your local machines and use any of the Internet tools to make use of those services. They all use the same TCP/IP protocols used on the Internet. For example, with FTP tools, you can transfer files among the machines on your network. With mail tools, you can send messages from one machine to another, and with a Web browser, you can access local Web sites that may be installed on a machine running its own Web servers. If you want to have one of your machines connected to the Internet or some other network, you can set it up to be a gateway machine. By convention, the gateway machine is usually given the address 192.168.1.1. With a method called IP masquerading, you can have any of the non-Internet machines use a gateway to connect to the Internet.
Numbers are also reserved for class A and class B non-Internet local networks. Table 38-5 lists these addresses. The possible addresses available span from 0 to 255 in the host segment of the address. For example, class B network addresses range from 172.16.0.0 to 172.31.255.255, giving you a total of 32,356 possible hosts. The class C network ranges from 192.168.0.0 to 192.168.255.255, giving you 256 possible subnetworks, each with 256 possible hosts. The network address 127.0.0.0 is reserved for a system's loopback interface, which allows it to communicate with itself, enabling users on the same system to send messages to each other.
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IPv4 Private Network Addresses |
Network Classes |
|---|---|
|
10.0.0.0 |
Class A network |
|
172.16.0.0 to 172.31.255.255 |
Class B network |
|
192.168.0.0 |
Class C network |
|
127.0.0.0 |
Loopback network (for system self-communication) |
The broadcast address allows a system to send the same message to all systems on your network at once. With IPv4 class-based IP addressing, you can easily determine the broadcast address using your host address: the broadcast address has the host part of your address set to 255. The network part remains untouched. So the broadcast address for the host address 192.168.1.72 is 192.168.1.255 (you combine the network part of the address with 255 in the host part). For CIDR IP addressing, you need to know the number of bits in the netmask. The remaining bits are set to 1 (see Figure 38-3). For example, an IP address of 192.168.4.6/22 has a broadcast address of 192.168.7.255/22. In this case, the first 22 bits are the network address and the last 10 bits are the host part set to the broadcast value (all 1s).
In fact, you can think of a class C broadcast address as merely a CIDR address using 24 bits (the first three segments) for the network address, and the last 8 bits (the fourth segment) as the broadcast address. The value 255 expressed in binary terms is simply 8 bits that are all 1s. 255 is the same as 11111111.
|
IP Address |
Broadcast Address |
IP Broadcast Number |
Binary Equivalent |
|---|---|---|---|
|
192.168.1.72 |
192.168.1.255 |
255 |
11111111 |
|
192.168.4.6/22 |
192.168.7.255/22 |
7.255 (last 2 bits in 7) |
1111111111 |
Some networks have a computer designated as the gateway to other networks. Every connection to and from a network to other networks passes through this gateway computer. Most local networks use gateways to establish a connection to the Internet. If you are on this type of network, you must provide the gateway address. If your network does not have a connection to the Internet, or a larger network, you may not need a gateway address. The gateway address is the address of the host system providing the gateway service to the network. On many networks, this host is given a host ID of 1: the gateway address for a network with the address 192.168.1 would be 192.168.1.1, but this is only a convention. To be sure of your gateway address, ask your network administrator.
Many networks, including the Internet, have computers that provide a Domain Name Service (DNS) that translates the domain names of networks and hosts into IP addresses. These are known as the network's domain name servers. The DNS makes your computer identifiable on a network, using only your domain name, rather than your IP address. You can also use the domain names of other systems to reference them, so you needn't know their IP addresses. You must know the IP addresses of any domain name servers for your network, however. You can obtain the addresses from your system administrator (often more than one exists). Even if you are using an ISP, you must know the address of the domain name servers your ISP operates for the Internet.
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