72

Chapter 3

IP Addressing

So, in our case, instead of having one network (172.16.0.0) with 65,534

available hosts numbers, we have 254 networks (172.16.1.0172.16.254.0)

with 254 available host numbers in each subnet.

We can calculate the number of hosts available on a subnet by using the

formula 2

*n*

2 = number of available host IPs

, where *n* is the number

of hosts bits (in our example, 8). The minus 2 (2) represents all host bits on

and all hosts bits off, which are reserved.

Similarly, the number of networks (or subnets) can be calculated with

nearly the same formula: 2

*n*

2 = number of available networks

, where

*n*

is the number of subnet bits (in our example, 8). So, with subnetting we

have balanced our need for available network and host numbers. However,

there may be instances where we need fewer host numbers on a particular

subnet and more host numbers on another. The 2 represents all subnet bits

on and all subnet bits off.

Let's extend our example to include a serial link between two routers, as

shown in Figure 3.1.

F I G U R E 3 . 1

IP address example

Since these are routers and not switches, each interface belongs to a dif-

ferent network. The interfaces need to share a network to talk. How many

IP numbers do we really need on the network interconnecting the two rout-

ers? We only need two IP numbers, one for each serial interface, as shown in

T A B L E 3 . 3

IP Address Example

Decimal

172

16

0

0

Binary

10101100

00010000

00000000

00000000

Decimal

255

255

255

0

Binary

11111111

11111111

11111111

00000000

172.16.10.1

**Network 172.16.10.0/24**

172.16.10.2

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