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An awfully naive tool that calculates the broadcast IP, network ID, and hosts size - given an IPv4 address and a netmask

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idoqo/ipgalc

Given an IPv4 address and a network prefix, ipgalc calculates the corresponding

  • Broadcast IP
  • Network ID
  • First Host
  • Last Host
  • Group Size and
  • Hosts Size

Usage

go run ./main.go --ip $IP --prefix-bit $PREFIX 
e.g
go run ./main.go --ip 127.0.0.1 --prefix-bit 24

How it works

ipgalc works by first splitting up the given IP address into the individual octets, and uses the prefix to resolve the correct subnet (as well as the subnet octets). Both octets are represented as integer arrays (of length 4), and the calculation is performed thus:

Network ID

Each element (called octet) in the target IP is bitwise ANDed with the element at same index in the subnet mask to give the element at that index in the network ID e.g., say the IP address is "127.0.0.1" and prefix is 24 (i.e /24), the subnet mask is "255.255.255.0", then:

networkID[0] = 127 & 255 // 127
networkID[1] = 0 & 255   // 0
networkID[2] = 0 & 255   // 0
networkID[3] = 1 & 0   // 0

which gives the network id as 127.0.0.0

Broadcast IP

Each octet in the target IP is bitwise ORed with the bitwise complement of the element at same index in the subnet mask. The (negative) result is added to 256 to wrap-around the value, giving the correct value (PS: I can't figure out why it needs the extra 256). Using the previous example, we get:

broadcastIP[0] = 256 + (127 | (^255))  // 127
broadcastIP[1] = 256 + (0 | (^255))  // 0
broadcastIP[2] = 256 + (0 | (^255)) // 0
broadcastIP[3] = 256 + (1 | (^255)) // 255

giving broadcast IP as 127.0.0.255.

First Host

// todo :/

Last Host

// todo :/

Group Size

// todo :/

Host Size

Host size represents the number of hosts possible in the subnet (without regards for the Network ID and the Broadcast). Ipgalc resolves it by calculating: 2 ^ (32 - $PrefixBits) and substracting 2 from the result. The host size for the above example would be:

total = 2 ^ (32 - 24) // 256
hostSize = total - 2 // 254

Resources

License

Do What the Fuck you Want to.

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An awfully naive tool that calculates the broadcast IP, network ID, and hosts size - given an IPv4 address and a netmask

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