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Subnetting: A way to divide a network into smaller, manageable parts

An introduction to subnetting, why it is used, how it works, and how to calculate subnets and hosts.

15 min read
NetworkingSubnettingIP AddressNetwork Management

Subnetting: A way to divide a network into smaller, manageable parts.

Subnetting is the way we break down a big network into smaller pieces. This helps us manage the network better, improve security, enable efficient routing, and ensure that data gets to the right place.

History of Subnetting

Subnetting was introduced in the 1980s as a way to improve the efficiency of IP address allocation. Before subnetting, large organizations often wasted many IP addresses because they were assigned a whole class of addresses (like Class A, Class B, or Class C) even if they only needed a few. Subnetting allowed these organizations to divide their networks into smaller, more efficient segments.

Why Subnetting?

Subnetting divides a big network into smaller parts called subnets. This helps in:

  • Better management of the network.
  • Improved security by isolating parts of the network.
  • Efficient use of IP addresses.
  • Reduced network traffic.
  • Easier troubleshooting.

How Subnetting Works

When we subnet a network, we take the original IP address and divide it into two parts: the network part and the host part. The network part identifies the subnet, while the host part identifies individual devices within that subnet.

To create subnets, we use a subnet mask. The subnet mask determines how many bits are used for the network part and how many are used for the host part. By changing the subnet mask, we can create multiple subnets from a single network.

Subnetting Math

To calculate the number of subnets and hosts, use the following formulas:

  • Number of subnets: 2^n
    • n is the number of bits borrowed from the host portion of the original IP address to create subnets.
  • Number of hosts per subnet: 2^h - 2
    • h is the number of bits remaining for hosts.
    • Subtract 2 to account for the network and broadcast addresses, which cannot be assigned to hosts.

Example of Subnetting

For example, if we have a Class C network with the IP address 192.168.1.0 and a default subnet mask of 255.255.255.0, we can create smaller subnets by borrowing bits from the host part. If we borrow 2 bits, we can create 4 subnets, each with 62 usable host addresses. It looks like this:

TEXT
Subnet 1: 192.168.1.0/26 (usable IP range: 192.168.1.1 - 192.168.1.62)
Subnet 2: 192.168.1.64/26 (usable IP range: 192.168.1.65 - 192.168.1.126)
Subnet 3: 192.168.1.128/26 (usable IP range: 192.168.1.129 - 192.168.1.190)
Subnet 4: 192.168.1.192/26 (usable IP range: 192.168.1.193 - 192.168.1.254)
Subnetting Diagram

CIDR (Classless Inter-Domain Routing)

CIDR is a method for allocating IP addresses and routing that replaces the old system based on classes (like Class A, Class B, Class C). CIDR allows for more flexible subnetting by using VLSM (variable length subnet masks). This means that networks can be divided into subnets of different sizes based according to the specific needs of the organization.

Routing Between Subnets

  • Routers are used to connect different subnets. They help direct traffic between subnets and ensure that data reaches its intended destination.
  • Each subnet has its own unique network address, and routers use this information to forward packets appropriately.
  • Routing protocols like OSPF and EIGRP can be used to manage the routing between subnets efficiently.

Real World Use

Subnetting is widely used in businesses and organizations to manage their networks. For example, a company might have different subnets for different departments, such as HR, IT, and Sales. This way, they can control access to sensitive information and reduce network congestion.