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IP Subnetting Step #4: Determining Subnet Identifiers and Subnet Addresses
(Page 3 of 5)
Class B Subnet ID and Address Determination Example
Let's look at our other example now,
Class B network 18.104.22.168. In binary this is:
0100110 01110001 00000000
We are using 5 bits for the subnet
ID, leaving 11 host ID bits. The network address with the subnet ID
bits highlighted is:
0100110 01110001 00000000
Here, only the third octet will ever
change for the different subnets. The first two will always be 166.113
and the last octet will always be zero. There are 32 possible subnets;
I'll list the first few so you can see the pattern (refer to Figure 79
0. Subnet #0 has a subnet ID of 00000.
This means the address will be 22.214.171.124, the network address, as
we would expect.
- Subnet #1 has a subnet ID of 00001.
The address becomes:
10100110 01110001 00001000
This is 126.96.36.199 in
- Subnet #2 has a subnet ID of 00010,
giving an address of 116.113.00010000.0
- Subnet #3 has a subnet ID of 00011
and a subnet address of 188.8.131.52.
Figure 79: Determining Subnet Addresses For A Class B Network
This is the same as Figure 78, but for a Class B network with 5 subnet ID bits (I have not shown all 32 subnets, for obvious reasons!)
Again, the pattern here is obvious:
you add 8 to the third octet to get successive addresses. The last subnet
here is #31, which has a subnet address of 184.108.40.206, which has
the same third and fourth octets as our subnet mask of 255.255.248.0.
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Version 3.0 - Version Date: September 20, 2005
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