# What if the subnet mask is 255.255.255.0 ?CompTIA Network+ Course

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This topic contains 3 replies, has 3 voices, and was last updated by  Paul Rouk 3 years, 4 months ago.

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• #78905

newen
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increment = 256 – 0 ?????????

#79057

Brian Idol
Participant

Not real sure what you are asking. The address 255.255.255.0 is the default subnet mask for class C networks. If your IP address begins with 192 or above then you are not subnetting.

However if the first octet of your IP address is 191 or lower then you are subnetting. To understand this a little better lets look at the class ranges.

Class A IPs are anything that begins between 1 and 127. The default subnet mask for Class A is 255.0.0.0
Class B IPs are between 128 and 191. Default subnet mask is 255.255.0.0
Class C are 192-223. Default mask is 255.255.255.0

So what does that mean? If you have a class B network (say 172.22.0.0) and the default mask of 255.255.0.0, then you are using 16 bits for the network address and 16 bits for the host address. 1 network (172.22.0.0) with 2^16 individual host addresses (65536 addresses). The problem with this is that we only have 1 broadcast address, so if someone pings that address it will go to all 65536 hosts and create a lot of network congestion. So what can we do? That is where subnetting comes in.

Not sure if that answered your question or not, but hopefully it helps you out some. Let me know if you’re still confused and I’ll help you the best I can.

#79150

Paul Rouk
Participant

There are a lot of free IP subnet calculators available. You can play around with some of them until you get a better feel for how subnetting works, but the goal is that you should be able to do the subnet calculations yourself without using a calculator. One sample calculator is at the link below.

https://www.tunnelsup.com/subnet-calculator/ — For example, try 192.168.1.1/24 where /24 is the number of bits used by the subnet mask (ie, 255.255.255.0)

#85582

Paul Rouk
Participant

I’ve tried a lot of graphical IP subnet calculators, but I just discovered one which runs in the Linux terminal. The program is called “sipcalc.” It is easy to install and provides a lot of useful information.

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