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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Subnetting: Practical Subnet Design and Address Determination Example

IP Subnetting Step #3: Determining The Custom Subnet Mask 1 2 3 4 5 IP Subnetting Step #5: Determining Host Addresses For Each Subnet

Class B Subnet ID and Address Determination Example

Let's look at our other example now, Class B network 166.113.0.0. In binary this is:

0100110 01110001 00000000 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 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 as well):

0. Subnet #0 has a subnet ID of 00000. This means the address will be 166.113.0.0, the network address, as we would expect.

10. Subnet #1 has a subnet ID of 00001. The address becomes:

10100110 01110001 00001000 00000000

This is 116.113.8.0 in decimal.

11. Subnet #2 has a subnet ID of 00010, giving an address of 116.113.00010000.0 or 116.113.16.0.
    
    
12. Subnet #3 has a subnet ID of 00011 and a subnet address of 116.113.24.0.
    

    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 116.113.248.0, which has the same third and fourth octets as our subnet mask of 255.255.248.0.

IP Subnetting Step #3: Determining The Custom Subnet Mask 1 2 3 4 5 IP Subnetting Step #5: Determining Host Addresses For Each Subnet