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 2 years, 3 months ago.

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  • #78905

    newen
    Participant

    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.

    In the example you gave we are still using the 172.22.0.0 network address but we change the subnet mask to 255.255.255.0. So what does this do for us? By changing the 3rd octet, we are telling the system that we want to use those 8 bits for the network address as well. So what does that do? Well 2^8 is 256. Meaning that we will have 256 different sub networks (172.22.1.0, 172.22.2.0, 172.3.0,……172.22.255.0). Each of these networks will have 8 bits (4th octet) for host addressing. Again 2^8 is 256, so we will have 256 addresses per network. However each network will need both a network address and a broadcast address. The network address is always the first available IP address on a network while the broadcast address is always the last available. So on the 172.22.1.x network, the network address will be 172.22.1.0 while the broadcast address is 172.22.1.255. With that said, there will only be 254 usable addresses per network.

    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.

    https://www.cyberciti.biz/faq/linux-subnet-calculator-cidr/

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