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IP Addressing
19
Administration for Network Connectivity
555-233-504 — Issue 1 — April 2000 CID: 77730
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.
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.
Binary
Subnet
Boundaries
(for 3 bits)
Decimal
Subnet
Boundaries
Range of usable IP
Addresses in the
Subnet
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