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My question is not a duplicate "The slash after an IP Address - CIDR Notation" because the answers to this question say that CIDR is our SUBNET MASK. And My question is "If CIDR is our subnet mask - why am I provided with second subnet mask?"

So I am given bunch of IP/CIDR and subnet masks and I am asked to calculate # of subnets and hosts per subnet. For example

IP address ---- Subnet mask ----- Number of subnets ----- Number of hosts per subnet

128/8 ---------255.255.255.128 ------------------ ? -------------------------------- ? ---------------

What I am confused about is what does it mean to give subnet mask if we are given CIDR. I though CIDR is exactly subnet mask but represented with different notation.

From wiki: 192.168.100.14/24 represents the IPv4 address 192.168.100.14 and its associated routing prefix 192.168.100.0, or equivalently, its subnet mask 255.255.255.0, which has 24 leading 1-bits.

So I though if I have 128/8 then I have 255.0.0.0 as a subnet mask. But then I am given another subnet mask. How should I treat this?

Thanks

YohanRoth
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1 Answers1

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The question is asking you to take the large network (with the slash notation) and divide it up into smaller subnets, each one using the given subnet mask.

So if you have a network 128/8 (or more correctly, 128.0.0.0/8), and you want to subdivide it into smaller subnets using the mask 255.255.255.128, how many subnets can you make? And how many hosts are in each of those subnets?

Note that this problem could be asked using all slash notation, or all dotted decimal. The problem uses both just to make sure you are proficient with both.

Ron Trunk
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