Subnetting (and Drills)
Subnetting Cheat Sheet**:
128 64 32 16 8 4 2 1
128 192 224 240 248 252 254 255
/25 /26 /27 /28 /29 /30 /31 /32
/17 /18 /19 /20 /21 /22 /23 /24
/9 /10 /11 /12 /13 /14 /15 /16
/1 /2 /3 /4 /5 /6 /7 /8
Using that cheatsheet, solve for:
Network ID:
First Usable IP:
Last Usable IP:
Broadcast ID:
Next Network:
Subnet Mask:
Number of Hosts:
Subnetting practice/drills: https://subnetipv4.com/
With subnet mask practice/drills: https://richtechguy.com/subnetting-practice-tool/
CIDR Calculator: https://cidr.xyz/
Networking Classes
| Class | 1st Octet Range | Subnet Mask | Description |
|---|---|---|---|
| A | 1–126 | 255.0.0.0 | Designed for a small number of networks with a very large number of hosts on each one. |
| B | 128–191 | 255.255.0.0 | Used for medium-sized networks. |
| C | 192–223 | 255.255.255.0 | Used for a large number of networks with a small number of hosts on each one. |
| D | 224–239 | N/A | Reserved for multicast traffic. |
| E | 240–255 | N/A | Reserved for experimental use. |
How Many Possible
All of the possible hosts, not just usable (eg, not subtracting network and broadcast).
- 4th octet: 256 (255.255.255.0)
- 3rd octet: 65,536 (255.255.0.0)
- 2nd octet: 16,777,216 (255.255.0.0)
- 1st octet: 4,294,967,296 (0.0.0.0), all of them
If you want to see how many are in a 255.240.0.0, you'd multiple the number leftover in the 2nd octet (256-240 = 16) x 65536, because each of those 16 could be a network as big as 65536.
For 255.255.240.0, that would be 16 x 255.
Useful Linux app:
ipcalc --split 2 $(curl -s ha17.com/ip)
Might need:
sudo apt update && sudo apt install ipcalc
ipv6 goes up to /128. To get the number of hosts: 2^(128 - n) where n=CIDR slash number
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Private Network Ranges
- 10.0.0.0 to 10.255.255.255 (Class A): This range offers over 16 million IP addresses and is typically used for large networks, like large corporations.
- 172.16.0.0 to 172.31.255.255 (Class B): This range provides more than one million IP addresses and is suitable for medium-sized networks, such as schools or medium-sized businesses.
- 192.168.0.0 to 192.168.255.255 (Class C): This range provides over 65,000 IP addresses and is commonly used for home and small office networks.
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** I needed this a lot at first but suddenly find it pretty easy to do this from memory.