Networking Concepts

Subnetworks

Figure 2-2 Class C Address with Subnet Number

Now, refer again to example 1. The subnet mask must indicate that three bits of the node portion of the IP address will be used for the subnet identifier. The subnet mask turns on (sets to 1) all the relevant bits for its subnet scheme. The subnet mask for example 1 is shown below. Note that the most significant three bits of the rightmost byte are set.

Subnet Mask

Binary 11111111.11111111.11111111 11100000

Decimal 255.255.255 224

Table 2-1shows valid addresses for the subnetwork in example 1. You will need to know this information for NMMGR configuration. The table shows the possible values of the rightmost byte of the IP address for each of the subnets, given the criteria described in the example. (Remember, an address of all 0s or all 1s is not valid).

Column 2 shows the values, in binary, of the six subnet addresses. Five zeroes are shown in parentheses to indicate where the three subnet-address bits are located in the byte. The equivalent decimal value for each subnet address is shown in the third column. The fourth column shows the range of possible values for the node address of each subnet. The five rightmost bits make up the node portion, and the range is the same for all subnets. By combining the subnet address with the range of node addresses, the possible decimal values of the rightmost byte are obtained and shown in the fifth column.

The table shows that subnets of 30 nodes each are possible given a subnet mask of 255.255.255 224. This is derived from the column that shows the range of possible values for the five bits that make up the node portion of the IP address. The range for each of the six subnets shows 30 possible values.

Chapter 2

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