Foundation Summary

Study Material For Cisco Ccna Ccnp And Ccie Students

Study Material For Cisco Students

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The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your CCNA exam(s), a well-prepared CCNA candidate should know, at a minimum, all the details in each "Foundation Summary" section before going to take the exam.

The thought process used to answer questions about the number of hosts and subnets in a network, based on a network number and a subnet mask, is summarized in the following list:

Step 1 Identify the structure of the IP address.

Step 2 Identify the size of the network part of the address, based on Class A, B, and C rules.

Step 3 Identify the size of the host part of the address, based on the number of binary 0s in the mask.

Step 4 The size of the subnet part is what's left over; mathematically, it is 32 - (number of network + host bits).

Step 5 Declare the number of subnets, which is 2number-of-subnet-bits - 2.

Step 6 Declare the number of hosts per subnet, which is 2number-of-host-bits - 2.

A formal definition of the "algorithm" to find the first and last IP addresses in a subnet when the subnet number and broadcast addresses are known is as follows:

■ For the first valid IP address: Copy the subnet number, but add 1 to the fourth octet.

■ For the last valid IP address: Copy the broadcast address, but subtract 1 from the fourth octet.

■ The range of valid IP addresses starts with the first number and ends with the last.

To find the subnet number, perform a Boolean AND between the address and the subnet mask. For example, see Table 12-38.

Table 12-38 Bitwise Boolean AND Example




1001 0110 1001 0110 0000 0010 0000 0001


1111 1111 1111 1111 1111 1111 0000 0000

Result of AND

1001 0110 1001 0110 0000 0010 0000 0000

To find the subnet broadcast address, take the subnet number in binary and change all the host bits to binary 1s.

The following three-step process lists all the subnet numbers of a network. The process refers to the chart that follows the three-step list:

Step 1 Write down the network number and subnet mask in the first two rows of the subnet list chart.

Step 2 Write down the network number in the third row. This is the zero subnet, which is one of the two reserved subnets.

Step 3 Do the following two tasks, stopping when the next number that you would write down in the interesting column is 256. (Don't write that one down—it's not valid.)

a. Copy all three noninteresting octets from the previous line.

b. Add the magic number to the previous interesting octet, and write that down as the value of the interesting octet. (See Table 12-39).

Table 12-39 Subnet List Chart—






Network number


Subnet zero

First subnet

Next subnet

Skip a lot

Last subnet

Broadcast subnet

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