Suppose you are given an IPv4 CIDR address
153.10.22.56 /22
This IP address represents a particular device (“host”, in networking terminology) on a network, such as a laptop computer or gaming system.
From this simple set of numbers you can calculate all sorts of things about the network it belongs to, such as the network’s starting address, the number of devices (“hosts”, in networking terminology) the network supports, and the ordinal number in the network of this particular device. Let’s go be leet hackers…
Find the Network mask
The number after the “/” indicates what portion of the network address belongs to the network itself, and which portion is available for hosts. In the case of /22, it means the first 22 bits are used for the network itself, and the remaining 10 bits can be used by the various hosts that join the network.
In a 32-bit binary number, the first 22 bits should be 1’s, and the rest 0’s.
11111111 11111111 11111100 00000000 // that's 22 1's
Convert each “chunk” back to decimal and you get:
255.255.252.0 // the netmask!
Find the Network’s Address
Now that we know the network mask, we can figure out the network’s address. The network address is the address of the network itself and it is the lowest address on the network.
If you’ve ever poked around your own home network, you might recognize 192.168.1.0 (or similar). All devices that join the network are given a similar, higher-numbered IP address when they are on the network: your phone might be 192.168.1.05, your laptop might be 192.168.1.12, etc.
You can find the network’s address by using your knowledge of the IP address of any given device on the network and the netmask.
Start by converting the IP address and the netmask to binary. Then, perform a bitwise and, meaning that for any position that has two 1’s, the result will be a 1. (Two zeroes or a 1 and a zero results in a 0.)
153.10.22.56 10011001 00001010 00010110 00111000
255.255.252.0 11111111 11111111 11111100 00000000
--------------------------------------
Bitwise AND: 10011001 00001010 00010100 00000000
Convert the result of the bitwise and back to decimal to get your network address:
153.10.20.0 // the network address!
Sanity check: The network address is always the lowest address on the network, so it makes sense that a device found at 153.10.22.56 could be somewhere in a network that began its addresses at 153.10.20.0. You should be suspicious of your result if you end up with a network address that is higher than the IP you were given to start with.
Find the Host Mask
The host mask is the inverse of the network mask and we’ll use it later on to find the broadcast address.
Go back to your network mask result in binary and flip the 1’s and 0’s to get the host mask:
11111111 11111111 11111100 00000000 // network mask 00000000 00000000 00000011 11111111 // host mask (bits flipped)
You may wish to convert it to decimal:
0.0.3.255
We’ll use the host mask in the next section to find the broadcast address.
Sanity check: Since the host mask is just the opposite of the network mask, you might able to “eyeball it” by finding the difference between each portion of the address and 255. For the given netmask, 255.255.252.0, you could subtract 255-255 = 0 for the first two parts, 255-253 = 3 for the second-to-last part, and 0-255=|-255| for the last part. Be suspicious of any result you get that doesn’t appear to be the “inverse” of the net mask you started with.
Find the Broadcast Address
The broadcast address is the highest address on the network.
To find it, use the IP address you started with and the host mask from the previous step. Perform a bit mask on them, meaning that anywhere you have a 1 is good enough to get a 1 in the result. Two 1’s = 1 and a 0 and a 1 = 1.
153.10.22.56 10011001 00001010 00010110 00111000 // IP
0.0.3.255 00000000 00000000 00000011 11111111 // host mask
--------------------------------------
Bit mask: 10011001 00001010 00010111 11111111 // broadcast addr.
Convert to the broadcast address to decimal:
153.10.23.255 // the broadcast address!
Find the number of possible hosts
To find the maximum number of hosts a network can support, go back to that “/22” from the IP address you started with. Subtract it from 32. (Why 32? Because that’s how many bits are in an address, total.)
32 - 22 = 10
Now raise 2 to that power:
2^10 = 1024
And subtract 2:
1024 - 2 = 1022 // hopefully that's enough room for everyone
We subtract 2 because there are two reserved host numbers: the network address itself, and the broadcast address. We can’t assign those to laptops and smart TVs so we don’t include them in the maximum number of supported hosts.
Find the host number
My class often asked us for the “host number” of a device but didn’t really explain what was meant by that or how to calculate it.
The host number is just the ordinal number of the host on the network. It’s the Nth device on the network. Pretend you’ve already got a printer, a TV, and a computer on your network. You add your phone. Your phone is the 4th device to join. Its host number is 4.
In other words, you’re not looking for an IP address here, you’re looking for an ordinal number. For the sake of questions like the ones I got in CS372, you’re also assuming that hosts were added “in order” and assigned their IP addresses accordingly.
You need to know both the network’s address and the IP of the device itself. it from the IP if you know where the network starts.
153.10.22.56 // this is where we are, what is our host number? 153.10.20.0 // this is where the network starts
I imagine each section of the IP address to be like a bucket. Each bucket holds 255. Once a 256th is added, the number to its left has to increase by 1 and that bucket is reset to 0. (This is just like “carrying over” a number the way we’ve done since elementary school.)
Let’s do an easy one first.
153.10.20.0 // this is where the network starts 153.10.20.1 // what host number am I?
The device at 153.10.20.1 is host number 1. It’s the first host on the network because 153.10.20.0 is reserved as the network’s own address.
Here’s a harder one:
153.10.20.0 // this is where the network starts 153.10.20.255 // what host number am I?
The device at 153.10.20.255 is host number 255.
Now let’s see what happens when the number to the left also changed.
153.10.20.0 // this is where the network starts 153.10.21.1 // what host number am I?
Hey, that 20 became a 21! We must have added a lot of hosts to this network. We added so many, that the 153.10.20.nnn space was exhausted and we had to increase 20 to 21 and start numbering over. This “carry over” is just like you did in elementary school math, except here the limit is 255.
Since this is where it gets complicated, I’ll show you the steps:
153.10.20.0 // this is where the network starts
153.10.21.1 // what host number am I?
20 - 21 = 1 // there was 1 "carry over" 1 * 255 = 255 // which means 255 hosts were added... 255 + 1 = 256 // and there's still 1 in the far-right bucket 256 + 1 = 257 // add that 1 from the first line to account for reserved addresses
The device at 153.10.21.1 is host number 257.
Hopefully you see the pattern here: each section is like a “bucket” that can be filled to 255, and then it has to “carry over” to the left and get reset to 0.
Now that you know how it’s calculated, we can look at “reverse engineering” it from our problem’s IP address.
153.10.20.0 // this is where the network started 153.10.22.56 // this is where we are, what is our host number?
22 - 20 = 2 // two "carry overs" in the bucket 2 * 255 = 510 // 510 were added to get from 20.0 to 22.0 510 + 56 = 566 // add the 56 from the last bucket 566 + 2 = 568 // add 2 from the first line
The device at 153.10.22.56 is host number 568.
I think that last step is worth elaborating on a bit more. The 2 was added because 153.10.21.0 and 153.10.22.0 are also not used for devices. The 2 was derived from subtracting 20 from 22 (the second-to-last segment of the IP address).
More host number examples
Example 1:
128.193.42.0 // this is where the network starts 128.193.43.35 // this is where we are, what is our host number?
43 - 42 = 1 // there was one "carry over" 1 * 255 = 255 // so we know at least that many hosts were added so far 255 + 35 = 290 // add the 35 from the furthest-right bucket 290 + 1 = 291 // add that 1 from the first step, we're host #291
Example 2:
128.193.0.0 // this is where the network starts 128.193.43.35 // this is where we are, what is our host number?
43 - 0 = 43 // that's a lotta "carry overs" 43 * 255 = 10965 10965 + 35 = 11000 11000 + 43 = 11043 // we're host #11043
That was absolutely amazing and simple!