1、Understanding TCPIP addressing and subnetting basics INTRODUCTION When you configure the TCP/IP protocol on a Microsoft Windows computer, an IP address, subnet mask, and usually a default gateway are required in the TCP/IP configuration settings. To configure TCP/IP correctly, it is necessary to und
2、erstand how TCP/IP networks are addressed and divided into networks and subnetworks. This article is intended as a general introduction to the concepts of IP networks and subnetting. A glossary is included at the end of article. Back to the top MORE INFORMATION The success of TCP/IP as the network p
3、rotocol of the Internet is largely because of its ability to connect together networks of different sizes and systems of different types. These networks are arbitrarily defined into three main classes (along with a few others) that have predefined sizes, each of which can be divided into smaller sub
4、networks by system administrators. A subnet mask is used to divide an IP address into two parts. One part identifies the host (computer), the other part identifies the network to which it belongs. To better understand how IP addresses and subnet masks work, look at an IP (Internet Protocol) address
5、and see how it is organized. Back to the top IP addresses: Networks and hosts An IP address is a 32-bit number that uniquely identifies a host (computer or other device, such as a printer or router) on a TCP/IP network. IP addresses are normally expressed in dotted-decimal format, with four numbers
6、separated by periods, such as 192.168.123.132. To understand how subnet masks are used to distinguish between hosts, networks, and subnetworks, examine an IP address in binary notation. For example, the dotted-decimal IP address 192.168.123.132 is (in binary notation) the 32 bit number 1100000001010
7、00111101110000100. This number may be hard to make sense of, so divide it into four parts of eight binary digits. These eight bit sections are known as octets. The example IP address, then, becomes 11000000.10101000.01111011.10000100. This number only makes a little more sense, so for most uses, con
8、vert the binary address into dotted-decimal format (192.168.123.132). The decimal numbers separated by periods are the octets converted from binary to decimal notation. For a TCP/IP wide area network (WAN) to work efficiently as a collection of networks, the routers that pass packets of data between
9、 networks do not know the exact location of a host for which a packet of information is destined. Routers only know what network the host is a member of and use information stored in their route table to determine how to get the packet to the destination hosts network. After the packet is delivered
10、to the destinations network, the packet is delivered to the appropriate host. For this process to work, an IP address has two parts. The first part of an IP address is used as a network address, the last part as a host address. If you take the example 192.168.123.132 and divide it into these two par
11、ts you get the following: 192.168.123. Network .132 Host -or- 192.168.123.0 - network address. 0.0.0.132 - host address. Back to the top Subnet mask The second item, which is required for TCP/IP to work, is the subnet mask. The subnet mask is used by the TCP/IP protocol to determine whether a host i
12、s on the local subnet or on a remote network. In TCP/IP, the parts of the IP address that are used as the network and host addresses are not fixed, so the network and host addresses above cannot be determined unless you have more information. This information is supplied in another 32-bit number cal
13、led a subnet mask. In this example, the subnet mask is 255.255.255.0. It is not obvious what this number means unless you know that 255 in binary notation equals 11111111； so, the subnet mask is: 11111111.11111111.11111111.0000000 Lining up the IP address and the subnet mask together, the network an
14、d host portions of the address can be separated: 11000000.10101000.01111011.10000100 - IP address (192.168.123.132) 11111111.11111111.11111111.00000000 - Subnet mask (255.255.255.0) The first 24 bits (the number of ones in the subnet mask) are identified as the network address, with the last 8 bits
15、(the number of remaining zeros in the subnet mask) identified as the host address. This gives you the following: 11000000.10101000.01111011.00000000 - Network address (192.168.123.0) 00000000.00000000.00000000.10000100 - Host address (000.000.000.132) So now you know, for this example using a 255.25
16、5.255.0 subnet mask, that the network ID is 192.168.123.0, and the host address is 0.0.0.132. When a packet arrives on the 192.168.123.0 subnet (from the local subnet or a remote network), and it has a destination address of 192.168.123.132, your computer will receive it from the network and process
17、 it. Almost all decimal subnet masks convert to binary numbers that are all ones on the left and all zeros on the right. Some other common subnet masks are: Decimal Binary 255.255.255.192 1111111.11111111.1111111.11000000 255.255.255.224 1111111.11111111.1111111.11100000 Internet RFC 1878 (available
18、 from http:/ ) describes the valid subnets and subnet masks that can be used on TCP/IP networks. Back to the top Network classes Internet addresses are allocated by the InterNIC (http:/ ), the organization that administers the Internet. These IP addresses are divided into classes. The most common of
19、 these are classes A, B, and C. Classes D and E exist, but are not generally used by end users. Each of the address classes has a different default subnet mask. You can identify the class of an IP address by looking at its first octet. Following are the ranges of Class A, B, and C Internet addresses
20、, each with an example address: Class A networks use a default subnet mask of 255.0.0.0 and have 0-127 as their first octet. The address 10.52.36.11 is a class A address. Its first octet is 10, which is between 1 and 126, inclusive. Class B networks use a default subnet mask of 255.255.0.0 and have
21、128-191 as their first octet. The address 172.16.52.63 is a class B address. Its first octet is 172, which is between 128 and 191, inclusive. Class C networks use a default subnet mask of 255.255.255.0 and have 192-223 as their first octet. The address 192.168.123.132 is a class C address. Its first
22、 octet is 192, which is between 192 and 223, inclusive. In some scenarios, the default subnet mask values do not fit the needs of the organization, because of the physical topology of the network, or because the numbers of networks (or hosts) do not fit within the default subnet mask restrictions. T
23、he next section explains how networks can be divided using subnet masks. Back to the top Subnetting A Class A, B, or C TCP/IP network can be further divided, or subnetted, by a system administrator. This becomes necessary as you reconcile the logical address scheme of the Internet (the abstract worl
24、d of IP addresses and subnets) with the physical networks in use by the real world. A system administrator who is allocated a block of IP addresses may be administering networks that are not organized in a way that easily fits these addresses. For example, you have a wide area network with 150 hosts
25、 on three networks (in different cities) that are connected by a TCP/IP router. Each of these three networks has 50 hosts. You are allocated the class C network 192.168.123.0. (For illustration, this address is actually from a range that is not allocated on the Internet.) This means that you can use
26、 the addresses 192.168.123.1 to 192.168.123.254 for your 150 hosts. Two addresses that cannot be used in your example are 192.168.123.0 and 192.168.123.255 because binary addresses with a host portion of all ones and all zeros are invalid. The zero address is invalid because it is used to specify a
27、network without specifying a host. The 255 address (in binary notation, a host address of all ones) is used to broadcast a message to every host on a network. Just remember that the first and last address in any network or subnet cannot be assigned to any individual host. You should now be able to g
28、ive IP addresses to 254 hosts. This works fine if all 150 computers are on a single network. However, your 150 computers are on three separate physical networks. Instead of requesting more address blocks for each network, you divide your network into subnets that enable you to use one block of addresses on multiple physical networks. In this case, you divide your network into four subnets by using a subnet mask
