To create subnetworks, you take bits from the host portion of the IP address and reserve them to define the subnet address. This means fewer bits for hosts, so the more subnets, the fewer bits available for defining hosts.
1. Determine the number of required network IDs:
One for each subnet
One for each wide area network connection
2. Determine the number of required host IDs per subnet:
One for each TCP/IP host
One for each router interface
3.Based on the above requirement, create the following:
One subnet mask for your entire network
A unique subnet ID for each physical segment
A range of host IDs for each subnet
Understanding the Powers of 2
Powers of 2 are important to understand and memorize for use with IP subnetting. To review powers of 2, remember that when you see a number with another number to its upper right (called an exponent), this means you should multiply the number by itself as
many times as the upper number specifies. For example, 2
3 is 2×2×2, which equals 8.
Here’s a list of powers of 2 you should commit to memory:
21=2 22=4 23=8 24=16
25=32 26=64 27=128 28=256
Subnet Masks
For the subnet address scheme to work, every machine on the network must know which part of the host address will be used as the subnet address. This is accomplished by assigning a subnet
mask to each machine. A subnet mask is a 32-bit value that allows the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address.
The network administrator creates a 32-bit subnet mask composed of 1s and 0s. The 1s in the subnet mask represent the positions that refer to the network or subnet addresses.
Not all networks need subnets, meaning they use the default subnet mask. This is basically the same as saying that a network doesn’t have a subnet address. shows the default subnet
masks for Classes A, B, and C. These default masks cannot change. In other words, you can’t make a Class B subnet mask read 255.0.0.0. If you try, the host will read that address as invalid and usually won’t even let you type it in. For a Class A network, you can’t change the first byte in a subnet mask; it must read 255.0.0.0 at a minimum. Similarly, you cannot assign 255.255.255.255, as this is all 1s—a broadcast address. A Class B address must start with 255.255.0.0, and a Class C has to start with 255.255.255.0.
