The Purpose of Subnet Masks

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The Purpose of Subnet Masks

A TCP/IP address is like a calendar date. The IP address has two components; a network ID and a host ID. The network ID may include a subnet ID. A date also has several components, such as a month, a day, and a year.

The subnet mask is used by the internet layer to determine which part of the IP address is the network ID and which part is the host ID. The subnet mask also can be used to determine whether a subnet is defined and to find the ID of that subnet. Calendar dates are also represented with a numbering scheme that communicates which part of the date is the month, day, and year.

As Table 4.1 shows, dates are listed with the month first, then the day, then the year (in the United States, at least). If the year is in the 1900s, it is common to omit the first two digits in the year. Americans are so used to this type of date scheme that they rarely stop to think about it. The TCP/IP subnet mask specifies that the octets of the IP address marked as 255 are the network ID and octets marked by 0 are the host ID. When subnetting you will begin to see other numbers appear instead of just 0 and 255.

It’s important to understand the correct date scheme when interpreting a date. For example, in Europe the orders of months and days are reversed. The date 12/6/69 is 12 June 1969 in Europe, whereas in the United States this date is interpreted as December 6, 1969. Similarly, with subnets, an understanding of the TCP/IP addressing scheme is necessary to decipher the IP address into the components of network ID, subnet ID, and host ID.

With a TCP/IP address, the address always follows the same format of four octets separated by periods. You can define different subnet masks, however, so that the address is interpreted differently. In Table 4.2, the same IP address listed in Table 4.1 is used, but with a different subnet mask. The address now specifies a subnet.

The TCP/IP address and subnet mask are made up of four 8-bit octets that, for ease of use, are viewed in decimal format rather than binary format. However, the address and subnet mask are actually binary so that IP understands them.

Any part of the subnet mask with 1s specifies the network portion of the address; 0s in the subnet mask specify the host portion of the address. The 1s are always at the first of the subnet mask, because an IP address always specifies the network portion of the address first. The host ID is specified by the remaining numbers of the IP address, which correspond to the 0s at the end of the subnet mask. In a subnet mask, note that the 1s are always grouped together and the 0s are always grouped together. The subnet mask basically divides the IP address into two pieces: the network ID and the host ID. The subnet mask simply indicates how many of the higher-order bits are devoted to the network ID and how many of the lower-order bits are devoted to the host ID.

The subnet mask determines how many host IDs are available. In the example in Table 4.2, there is a maximum of 254 different hosts on the network 200.20.16 (200.20.16.1 through 200.20.16.254). If you want to have more hosts on one network, you have to use a different addressing scheme. For example, if you use a subnet mask of 255.255.0.0, the address is interpreted as shown in Table 4.3.

With this subnet mask, two octets are available for the host ID. Using two octets allows you to have (256*256)-2 (you cannot use all 0s or all 1s) hosts on the network 200.20.

As noted, there are two cases that are not allowed for the host ID, these are where all bits are set to either 1 or 0. In these two cases the addresses are interpreted to mean a broadcast address (all 1s) or “this network only,” (all 0s). Neither of these destinations is valid for a host ID. Thus, the number of valid addresses is (2^n)-2, where n is the number of bits used for the host ID.

The example in Table 4.2 has fewer combinations of network IDs (because only two octets are used for the network) than in the example in Table 4.1 (in which three octets are used for the network). Bear in mind that you cannot always chose the subnet mask that allows you the greatest number of host IDs. For example, if the hosts are on the Internet, you must use a certain set of IP addresses assigned by the Internet address assignment authority, InterNIC.

Because the number of IP addresses available today is limited, you usually do not have the luxury to choose an addressing scheme that gives exactly the combination of host and network ID you require. Suppose you are assigned the network ID 139.20 and have a total of 1,000 hosts on three remote networks. A Class B network using the default subnet mask of 255.255.0.0 only has one network (139.20) yet allows 65,534 hosts.

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