IP addresses
Published on 2015-04-16 20:38
Categories: Infrastructure
TL;DR;
A cheat sheet to IPv4 subnetting and other quirks. Common subnet configurations with CIDR and netmask notation.
Disclaimer
I’m no expert in IPv4 networking, I’ve simply learnt stuff as I needed it. This post is mostly a cheat sheet for me, when I need to setup some infrastructure.
Private networks
There are three private address spaces currently in use.
- 127.0.0.0/18 - Not exactly a private network, but a reserved range for loopback intefaces.
- 10.0.0.0/8 - 16,777,216 addresses
- 172.16.0.0/12 - 1,048,576 addresses
- 192.168.0.0/24 - 65,536 addresses
In networks ranging from /1 to /30, there are always two addresses that are unusable as host addresses. They are the first address and last address. Example, in a 192.168.1.0/24 subnet, the address 192.168.1.0 and 192.168.1.255 are reserved as network address and broadcast address.
Since addresses are divided into 4 groups and and 32 (The CIDR bit) is dividable by 4, we can form this rule of thumb:
- If CIDR is lower than 8, it changes all groups.
- If CIDR is lower than 16 but higher or equal to 8, it changes the last three groups.
- If CIDR is lower than 24 but higher or equal to 16, it changes the last two groups.
- If CIDR is higher or equal to 24, it changes the last group only.
Examples: A /8 network ranges from *.0.0.0 to *.255.255.255, a /16 network ranges from *.*.0.0 to *.*.255.255 and thus a /24 network ranges from *.*.*.0 to *.*.*.255.
Table of netmasks
| CIDR | Netmask | Hosts | Usable hosts | Comments |
|---|---|---|---|---|
| 1 | 128.0.0.0 | 2,147,483,648 | 2,147,483,646 | |
| 2 | 192.0.0.0 | 1,073,741,824 | 1,073,741,822 | |
| 3 | 224.0.0.0 | 536,870,912 | 536,870,910 | |
| 4 | 240.0.0.0 | 268,435,456 | 268,435,454 | |
| 5 | 248.0.0.0 | 134,217,728 | 134,217,726 | |
| 6 | 252.0.0.0 | 67,108,864 | 67,108,862 | |
| 7 | 254.0.0.0 | 33,554,432 | 33,554,430 | |
| 8 | 255.0.0.0 | 16,777,216 | 16,777,214 | Class A |
| 9 | 255.128.0.0 | 8,388,608 | 8,388,606 | |
| 10 | 255.192.0.0 | 4,194,304 | 4,194,302 | |
| 11 | 255.224.0.0 | 2,097,152 | 2,097,150 | |
| 12 | 255.240.0.0 | 1,048,576 | 1,048,574 | |
| 13 | 255.248.0.0 | 524,288 | 524,286 | |
| 14 | 255.252.0.0 | 262,144 | 262,142 | |
| 15 | 255.254.0.0 | 131,072 | 131,070 | |
| 16 | 255.255.0.0 | 65,536 | 65,534 | Class B |
| 17 | 255.255.128.0 | 32,768 | 32,766 | |
| 18 | 255.255.192.0 | 16,384 | 16,382 | |
| 19 | 255.255.224.0 | 8,192 | 8,190 | |
| 20 | 255.255.240.0 | 4,096 | 4,094 | |
| 21 | 255.255.248.0 | 2,048 | 2,046 | |
| 22 | 255.255.252.0 | 1,024 | 1,022 | |
| 23 | 255.255.254.0 | 512 | 510 | |
| 24 | 255.255.255.0 | 256 | 254 | Class C |
| 25 | 255.255.255.128 | 128 | 126 | |
| 26 | 255.255.255.192 | 64 | 62 | |
| 27 | 255.255.255.224 | 32 | 30 | |
| 28 | 255.255.255.240 | 16 | 14 | |
| 29 | 255.255.255.248 | 8 | 6 | |
| 30 | 255.255.255.252 | 4 | 2 | |
| 31 | 255.255.255.254 | 2 | 2 | |
| 32 | 255.255.255.255 | 1 | 1 |