![]() ![]() ![]() This is going to be equal to the number of 1’s in our classful mask subtracted from our subnet mask (as explained above). This argument is basically saying how many bits are reserved for our subnet IDs. The only thing you need to know here is the “prefixlen_diff” argument. This python library has a function called “subnets” which will do exactly what your question is asking. #How to find subnet magic number full#Therefore for the full address we pass in the number of 1’s from the classful subnet mask which is 8. Here we are using it only because it will allow us to easily calculate out our subnet IDs.ĬIDR notation looks like this (16.0.0.0/8) where the “/8” represents the number of 1’s in the mask. Then we create an ip_network object (part of the python ipaddress library) and initialize it using CIDR notation.ĬIDR stands for Classless Interdomain Routing and is used instead of classful (what we are using in your example) in the real world. First we calculate the number of 1’s in both of the net masks binary forms. #How to find subnet magic number code#The code is relatively straight forward, but there are a few things that might need explaining. # Print the addresses (note there are going to be a LOT of them) Output = list(fullAddress.subnets(prefixlen_diff=(subnetMaskBits-classfulMaskBits))) # Print out all possible subnets against our fulladdress with subnet mask ![]() # Create an ip_network object using our network address and in CIDR notationįullAddress = ipaddress.ip_network(networkID + "/" + str(classfulMaskBits)) SubnetMaskBits = bin(int(ipaddress.IPv4Address(subnetMask))).count("1")ĬlassfulMaskBits = bin(int(ipaddress.IPv4Address(classfulSubnetMask))).count("1") # Calculate how many 1's are set in the binary forms of both net masks # Here I've left your variables exactly as they were in case this format is a requirement. I will post the example code and then explain below. Using the ‘ipaddress’ library in python we can solve the issue of finding all possible subnets very easily. Implementing a Solution Using Python "ipaddress" Library For instance, if your network id and subnet id equate to 16.0.0.0, we can have hosts in the range of 16.0.0.1 to 16.0.127.255. We also know that each subnet can have 32,768 hosts. We then know, based on our network ID of 16.0.0.0 that our subnet ID’s range from 16.0.0.0 to 16.255.128.0. Thus we have 9 bits of subnet addressing space, which If we take 2^9, we get 512 possible subnet IDs. This means we want to calculate out how many of those 1s in the subnet mask are unique to the subnet mask and not included in your classful mask. cannot be used by subnets), we also know that the bits NOT set by the subnet mask are reserved for hosts (those last 15 0s at the end of the subnet mask). We know that the bits set by the classful subnet mask is reserved for your network ID (i.e. Then you have your “ Subnet Mask” which is telling you which bits make up your Subnet IDs versus your Host IDs.įinally you have your “ Network ID” which is showing us the actual prefix that will be used for all Subnet and Host IDs. Any bits that are used for the class cannot be used for your subnet or your host IDs (the host IDs being the actual individual nodes on your network). With that said, here is how you would break this information down given the example you provided.įirst is the “ Classful Subnet Mask” which would be used to determine how many bits of your IP address make up the class. It should be noted that classful IP addressing is not used anymore in real-world scenarios. You can read more about determining IPv4 class here: Each class is determined in the first octet (first 8 bits) of the IP address. There are 5 classes when dealing with IPv4 addresses (A, B, C, D, E). ![]() I can edit the answer as needed if you were looking for something else. Since your example includes information about classful subnetting I will assume you want to calculate out the possible number of subnets given an IPv4 class and a subnet mask. There are generally two types of IPv4 addressing methodologies, classful and classless. ![]()
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