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
go run ./main.go --ip $IP --prefix-bit $PREFIX
e.g
go run ./main.go --ip 127.0.0.1 --prefix-bit 24
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:
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
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.
// todo :/
// todo :/
// todo :/
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
- ipcalc: an online IP address calculator.
- Practical Networking on YouTube: Teaches a human-friendly way to do subnet magic.