CCNP CIT Exam Cram 2 (642-831) Authors: Odom S. Published year: 2003 Pages: 61/213

### Subnetting Class C Network Addresses

When subnetting a Class C network address, only the last 8-bit octet is available for defining the hosts . Subnet bits are read from left to right. A Class C subnet mask would be shown as 255.255.255.x. The x would represent the octet that defines the host. This means the subnet mask can be one of these values, depending on the number of node addresses you need in each subnet:

00000000 = 0 for 254 nodes in each subnet.

10000000 = 128 for 126 nodes in each subnet.

11000000 = 192 for 62 nodes in each subnet.

11100000 = 224 for 30 nodes in each subnet.

11110000 = 240 for 14 nodes in each subnet.

11111000 = 248 for 6 nodes in each subnet.

11111100 = 252 for 2 nodes in each subnet.

11111110 = 254 is not valid for any host addresses.

Because you need at least two bits for defining hosts, the Request For Comments (RFCs) provide a rule that the first subnet mask that you can legally use is 192, and the last one is 252.

#### The Magic Number

##### Table 4.2. The First Three Subnets using a 255.255.255.252 Mask

Valid Hosts

4

5, 6

7

8

9, 10

11

12

13, 14

15

To put the magic number into action, consider an example using the network address 207.212.78.x and the 252 subnet mask. If you were to address the hosts in the first network, the hosts addresses would be 207.212.78.5 and 207.212.78.6. A broadcast to the hosts would be addressed as 207.212.78.7.

#### Class B Subnetting

If you have mastered the Class C network subnetting using the fourth octet, subnetting a class B network using the third octet is relatively easy. Simply take the same magic number 256 and the third octet of the IP address and apply the same process as you used for the Class C network. The only difference is that you now have all the addresses in the fourth octet to use for hosts, and that will be a lot more hosts. A Class B network address has 16 bits available for host addressing. Fourteen bits are available for subnetting, since we must leave at least two bits for host addresses. Let's take a look at the available subnets for a Class B network:

• 255.255.128.0

• 255.255.192.0

• 255.255.224.0

• 255.255.240.0

• 255.255.248.0

• 255.255.252.0

• 255.255.254.0

• 255.255.255.0

• 255.255.255.128

• 255.255.255.192

• 255.255.255.224

• 255.255.255.240

• 255.255.255.248

• 255.255.255.252

The process for subnetting a Class B network is similar to the process used to subnet Class C networks. Use the same subnet numbers you used with Class C, but add a zero to the network portion and a 255 to the broadcast section in the fourth octet. Table 4.3 shows an example of subnetting using a Class B 255.255.252.0 subnet mask:

##### Table 4.3. Class B Subnetting for the 255.255.255.252 Mask

Subnet

First Host

Last Host

4.0

4.1

7.254

7.255

8.0

8.1

11.254

11.255

This means if we were using the Class B network of 133.1.x.x, our first host address would be addressed as 133.1.4.1 and the last 133.1.7.254. Broadcasts to the subnet would beaddressed to 133.1.7.255.

 CCNP CIT Exam Cram 2 (642-831) Authors: Odom S. Published year: 2003 Pages: 61/213