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Subnetting

To understand subnetting, we have to take a look at IP addresses.

Assume we're using IPv4

An IP address a 32 byte identifier for a device on a network.

It's made up of 4 octets, each of which is a number between 0 and 255. Therefore, IP addresses can range from 0.0.0.0 to 255.255.255.255.

In binary, this can be represented as:

00000000 00000000 00000000 00000000 to
11111111 11111111 11111111 11111111

An IP address is split into 2 parts, to identify the network and the host. Subnetting allows us to "borrow" bits from the host part of an IP address to identify the network.

This has several benefits:

  • Improve network performance
  • Reduce network congestion
    • Without subnetting, a broadcast will reach every device in the network
  • Better network security
    • Without subnetting, if a device is compromised, the entire network becomes visible

Subnet mask

A subnet mask determines which parts are the network and host portions of the IP address

Classes

IP addresses are split into 4 classes.

To differentiate between the classes, the leading bits of an IP address is set to a predefined value

Class A

Default subnet mask Leading bits (binary ) Network bits Host bits First octet range
255.0.0.0 0 7 24 0 - 127

Class B

Default subnet mask Leading bits (binary) Network bits Host bits First octet range
255.255.0.0 10 16 16 128 - 191

Class C

Default subnet mask Leading bits (binary) Network bits Host bits First octet range
255.255.255.0 110 24 8 192 - 223

Class D

Default subnet mask Leading bits (binary) First octet range
N/A, multicast 1110 224 - 239

Class E

Default subnet mask Leading bits (binary) First octet range
N/A, research 11110 240 - 247

Reserved IPs

  1. [Broadcast address] - Host bits all set to 1
    • This is the address that is used to send messages to all devices on the network.
  2. [Network ID] - Host bits all set to 0
    • This is the address that is used to identify the network.
  3. [Loopback IP] - 127.x.x.x
    • This is the address that is used to send messages to the device itself.

IP broadcast addresses

  1. Limited broadcast - 255.255.255.255
  2. Directed broadcast - Same as broadcast address, host bits all set to 1

IP prefixes

A prefixed IP address simply represents the network bits used after the IP address itself.

For example, a subnet mask 255.255.255.240 will have the prefix /28.

Calculations

Some helpful calculations

Network ID

<destination IP> AND <subnet mask>

Number of subnets

If all zeros and all ones are not allowed:

\[ 2 ^ {subnet bits} - 2 \]

Else:

\[ 2 ^ {subnet bits} \]

Number of hosts

\[ 2 ^ {host bits} - 2 \]

Practice

Given class C network address 195.63.10.0, how to create 10 subnets?

Firstly we need to find the amount of subnet bits

\[ 2 ^ {subnet bits} - 2 \geq 10 $$ $$ 2 ^ {4} \geq 10 \]

Therefore, subnet mask is

11111111 11111111 11111111 11110000

Which in decimal is

255.255.255.240

Number of usable hosts for subnet

\[ 2 ^ {4} - 2 = 14 \]
Last update: June 11, 2023
Created: June 11, 2023