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106 Chapter 4: IP Addressing and Subnetting

Note that the network parts (the first two octets in this example) all begin with 150.150, meaning that each of the six subnets is a subnet of Class B network 150.150.0.0.

With subnetting, the third part of an IP address—namely, the subnet—appears in the middle of the address. This field is created by “stealing” or “borrowing” bits from the host part of the address. The size of the network part of the address never shrinks. In other words, Class A, B, and C rules still apply when you define the size of the network part of an address.

However, the host part of the address shrinks to make room for the subnet part of the address. Figure 4-4 shows the format of addresses when subnetting is used.

Figure 4-4 Address Formats When Subnetting Is Used

8

 

24 – x

x

 

Network

 

Subnet

Host

Class A

 

 

 

 

 

 

16

 

16 – x

x

 

Network

 

Subnet

Host

Class B

 

 

 

 

 

 

 

 

24

 

8 – x x

 

 

Network

Subnet Host

Class C

 

 

 

 

 

 

Analyzing and Interpreting IP Addresses and Subnets

You shouldn’t be surprised to learn that IP addressing is one of the most important topics on both exams. You need a comfortable, confident understanding of IP addressing and subnetting for success on any Cisco certification. You should be prepared to answer questions about the following:

An interpretation of an address

Its network number

Its subnet number

The other IP addresses in the same subnet

The broadcast address

The other subnets that could be used if the same mask were in use In other words, you had better know IP addressing and subnetting!

Besides just answering questions on the CCNA exams, network engineers need to understand subnetting very well to do their jobs. Engineers who work with multiple networks must decipher IP addresses quickly, without running off to use a subnet calculator tool. For