Internet Protocol version 4 ¶

Internet Protocol version 4 (IPV4) addresses are 32-bits long addresses. Ex: 01001101001000011110000100000000. The bits in the IP address are actually separated into 2 parts:

• id_net 🏠: it is the same bits for every address in a network.
• id_host 💻: The remaining bits. Identify a network device. If there are only null bits (0), then this is the network address.

You can't guess the number of bits used by id_net. Either it is given by an organism (such as an ISP), or you are given the netmask.

Example 🔥: is 01001101001000011110000100000000 a network address? We are told that the fixed part is 17 bits long.

• id_net: 01001101001000011 (17 bits)
• id_host: 110000100000000 (32-17=15 bits)

The id_host has non-null bits, so it's not a network address.

This is the most-known representation of an IPV4 address. We are splitting ours 32 bits into 4 groups of 8 bits called bytes/octets. Then, convert them to decimal, and separate them with a dot 🤓.

• Start with a binary IP (e.g., 0101100010[...]01)
• End up with n.n.n.n (where each $n \in [0,\ 255]$)
• Example: 127.0.0.1

Given the following IP: 01001101001000011110000100000000

• Split it into four blocks: 01001101, 00100001, 11100001, and 00000000
• Convert each block to decimal: 77, 33, 225, and 0
• Separate them with a dot: 77.33.225.0

See also: Online tool to convert bin to IP and Encoding.

CIDR is a way to write/share an IP address along the number of bits of the fixed part. The syntax is IP/n, such as 192.168.0.0/12 🚀.

• IP is the network address
• n is the number of fixed bits

💀 Traditionally, before CIDR, n could only be 8, 16, or 24. They were called A-class, B-class, and C-class networks.

We have the address 01001101001000011110000100000001 (77.33.225.1). We were told that there are 24 fixed bits.

• We are extracting 24 bits from the address: 010011010010000111100001
• We are filling the missing bits with 0 (32-24=8): 01001101001000011110000100000000
• We are converting the bits to DDN: 77.33.225.0
• We are adding /24: 77.33.225.0/24

The CIDR notation is 77.33.225.0/24.

This IP address is called a mask, as it was not created to be assigned to a machine, but to find the number of fixed bits in another IP address.

• We are writing $n$ (=number of fixed bits) non-null bits (1)
• We are filling the remaining bits with null bits (0)

If we know that 77.33.225.0 has 24 fixed bits, then we will write 24 times "1", and 8 (32-24) times "0", giving us 11111111111111111111111100000000 which is 255.255.255.0.

It's even easier to find the netmask. Example with 77.33.128.0/17.

• Calculate $\frac{n}{8}$ ($\frac{17}{8}$ gives us $q=2$, $r=1$)
• Calculate $c=255-2^{8-r} + 1$ ($c=255-2^{8-1}=128$)
• The result is
  • $q$ times $255$ (2 times $255$)
  • 1 time $c$ (1 times $128$)
  • and $\min(4-q-1, 0)$ times $0$ (1 times $0$)
• Giving us $255.255.128.0$

This is also a mask for the same reason as the netmask. It is used to send a message to EVERY machine in the network.

We are simply filling the id_host with non-null bits (1).

Given the IP 77.33.225.0/24,

• id_net: 010011010010000111100001 (extract 24 bits)
• id_host: 11111111 (fill last 8 bits with 1)

Giving us 01001101001000011110000111111111 which is 77.33.225.255.

There are 3 ranges of IPV4 addresses that are "reserved" for internal use. They are also called private addresses.

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16