User`s guide

182 Appendix E Understanding IP Addresses and Subnet Masking
8.20.15.1 = 00001000. 00010100. 00001111. 00000001
255.0.0.0 = 11111111. 00000000.
00000000. 00000000
Network ID Host ID
 The network address = 00001000 = 8
 The host address = 00010100.00001111.00000001 = 20.15.1
E.4 Understanding Subnets
A subnetwork (or “subnet” for short) allows a single Class A, B, or C network to be divided (or
“subnetted”) into smaller pieces. If you divide a Class A, B, or C network into smaller
subnetworks, it lets you create a network of interconnecting subnetworks where each data link
on the network has a unique ID. Without subnets, only one network from a Class A, B, or C
network could be used, which is an unrealistic restriction for many organizations.
Instead of the classful 2-level hierarchy, subnetting supports a 3-level hierarchy where the
natural mask is extended using bits from the host address portion of the address to create a
subnetwork ID. As the following figure shows, the subnetwork ID consists of two parts:
 The subnet number and
 The host number on that subnet
Two-Level Classful Hierarchy: Network Address Host Address
Three-Level Classful Hierarchy: Network Address Subnet Number Host Number
According to Internet practices, the host-number field of an IP address cannot contain all 0 bits
or all 1 bits. The all-0s host number identifies the base network or subnetwork number, while the
all-1s host number represents the broadcast address for the network or subnetwork.
The following examples assume there are five bits in the host number field of each subnet
address. This means that each subnet represents a block of 30 host addresses (2 5 -2 = 30, note
that the 2 is subtracted because the all-0s and the all-1s host addresses cannot be used). The
hosts on each subnet are numbered 1 through 30.
To define the address assigned to Host #n of a particular subnet, place the binary representation
of n into the subnet's host number field. For example, to define the address assigned to Host #15
on Subnet #1, place the binary representation of 15 (011112 ) into the 5 bits of Subnet #1's host-
number field.