Lucent Technologies Release 8.2 manual Example

IP Addressing 1 Networking Overview

Example

To continue the example using a 3-bit subnet ID, assume a Class C network ID of 192.168.50.xxx. This network ID can provide 254 usable IP addresses, all on the same network — from 192.168.50.1 to 192.168.50.254. If we divide this network into 3-bit subnets, we will have 6 usable subnets with 30 usable IP addresses in each subnet. Note that we have lost 74 usable IP addresses in the process because we had to discard the all-ones and all-zeros subnet IDs (62 addresses) and host IDs (12 addresses). There is always a loss of usable IP addresses with subnetting.

The following table shows the subnet boundaries for the six subnets formed with 3 bits. The boundaries are the numbers formed by using all combinations of 3 bits as the highest-order bits in an octet (Columns 1 and 2) and then using these numbers in the 4th octet for the host IDs.

Binary	Decimal	Range of usable IP
Subnet	Subnet	Addresses in the
Boundaries	Boundaries	Subnet
(for 3 bits)
00000000	0	not usable
00100000	32	192.168.50.33 to
		192.168.50.62
01000000	64	192.168.50.65 to
		192.168.50.94
01100000	96	192.168.50.97 to
		192.168.50.126
10000000	128	192.168.50.129 to
		192.168.50.158
10100000	160	192.168.50.161 to
		192.168.50.190
11000000	192	192.168.50.193 to
		192.168.50.222
11100000	224	not usable

For example, the IP addresses 192.168.50.75 and 192.168.50.91 are on the same subnet but 192.168.50.100 is on a different subnet. This is illustrated in the following diagram where the subnet mask, 255.255.255.244 is used to compare the first 27 binary digits or each address.

Administration for Network Connectivity
555-233-504 — Issue 1 — April 2000	CID: 77730	19