244 Chapter 5: Network Protocols
·
Each host address in the network has a unique value in the host part of the address. (For
example, 130.1.1.1 and 130.1.1.2 are in the same network but can be assigned to two
different network interfaces.)
Subnetting creates a third part of the address, called the subnet field or subnet part. For example,
using network 150.150.0.0 again, assume that you want a third field called the subnet field.
Several assertions are true in this case:
·
The Class A, B, and C network field sizes cannot be changed; they remain as 8, 16, and
24 bits, respectively.
·
The IP address must still be 32 bits in length.
·
Therefore, to create a third field called the subnet part of the address, some of the bits
previously in the host part of the address are used.
The subnet part of an address identifies the different subdivisions of this network. An address
with a different value in the subnet field, as compared with a second address, is considered to
be in a different subnet. For example, examine the following three IP addresses that are part of
Table 5-13 and are valid addresses in Figure 5-21:
The example shows that the subnet field consists of bits 17 through 24 (the entire third byte).
150.150.2.1 and 150.150.2.2 are in the same subnet because they are in the same Class B
network and because their subnet fields have the same value (0000 0010). 150.150.4.2 is in a
different subnet of the same Class B network because the subnet field has a different value than
the first two addresses (0000 0100). 150.150.4.2 must be physically located with at least one IP
router between itself and 150.150.2.1 and 150.150.2.2.
Five Ways the Exam Will Test Your IP Addressing Knowledge
CCNAs deal with IP addressing concepts from many different perspectives every day. To pass
the CCNA exam, you'll need to demonstrate the ability to think about IP addressing from each
of the following perspectives:
1
Given an IP address and mask, what is the network/subnet number?
2
Given an IP address and mask, what is the network/subnet broadcast address?
Table 5-13
Subnet Part of Sample Addresses
Address in Decimal
Address in Binary
150.150.2.1
1001 0110 1001 0110 0000 0010 0000 0001
150.150.2.2
1001 0110 1001 0110 0000 0010 0000 0010
150.150.4.2
1001 0110 1001 0110 0000 0100 0000 0010
ch05.fm Page 244 Monday, March 20, 2000 5:06 PM