How to Planning IP Address Step 4

Step 4: Figure Out the Network Addresses

In step 4, we need to figure out the networks that we created with our new subnet mask. Since IP addressing is done in binary, network addresses will always increment in a multiple of something. We’ll use this to our advantage when figuring out what our network numbers are for our Class C network.

Remember that the network number has all of the host bits set to 0s.

Actually, we already know what this multiplier is: we figured this out in the second part of step 2, using the 2 power of 4 - 2 = 14 formula. The 14 value is the number of valid host values for a subnet; however, this is not the total number of addresses for the subnet.

The subnet also has a network and broadcast address, which is the reason the formula subtracts 2 since you can’t use these addresses for host devices. Therefore, in our example, each network has a total of 16 addresses, and is incremented by 16 from subnet-to-subnet.

There is another method of verifying your multiplying value. In a byte, you can have numbers ranging from 0 - 255, resulting in a total of 256 numbers.

For this verification, take the subnet mask decimal value in the interesting octet and subtract it from 256. The interesting octet is the octet that contains the network and host boundary. In our case, this is the fourth octet. Therefore, using this trick, 256 - 240 = 16. When you compare this number to the number in the last paragraph, you can be assured that you have done your math correctly.

Now that you have figured out the multiplier, write the very first network down, and then start adding 16 to the interesting octet. Table as below lists the subnet numbers for 192.168.1.0. In this table, notice something interesting concerning the last subnet: 192.168.1.240. The network number in the last octet matches the interesting octet in our subnet mask (240). This will always be true when you perform subnetting.

There is one important item to point out about subnetting. In the original RFC for subnetting, you were not allowed to use the first and last subnet.

For instance, in our example, we would not be able to use 192.168.1.0/28 and 192.168.1.240/28. However, today, assuming that your TCP/IP protocol stack supports subnet 0 (this refers to these two subnets–first and last), you can. You need to make sure, though, that each device on the segment that will have one of these subnets supports this function. In today’s age, this shouldn’t be an issue.

However, I typically use this subnet for addressing only networking devices, like management devices, which typically support subnet 0.