Subnetting Subnet Mask, CIDR, VLSM: Tutorial, Examples, FAQs & Interview Tips

Why Subnetting?

Subnetting is the process of dividing a large network into smaller, more manageable sub-networks (subnets). Benefits include:

Reduces network congestion (smaller broadcast domains)
Improves security (isolate network segments)
Efficient use of IP address space
Easier network management and troubleshooting

Subnet Mask and CIDR Notation

A subnet mask is a 32-bit number that separates the network portion from the host portion of an IP address. It uses 1s for the network bits and 0s for the host bits.

CIDR (Classless Inter-Domain Routing) notation expresses the subnet mask as a prefix length (number of 1 bits).

CIDR	Subnet Mask	Network Bits	Host Bits	Usable Hosts
/8	255.0.0.0	8	24	16,777,214
/16	255.255.0.0	16	16	65,534
/24	255.255.255.0	24	8	254
/25	255.255.255.128	25	7	126
/26	255.255.255.192	26	6	62
/27	255.255.255.224	27	5	30
/28	255.255.255.240	28	4	14
/29	255.255.255.248	29	3	6
/30	255.255.255.252	30	2	2

Formula: Usable hosts = 2ⁿ - 2, where n = number of host bits. We subtract 2 for the network address and broadcast address.

Subnetting Example: /24 Network

Given network: 192.168.1.0/24

Network Address: 192.168.1.0 (all host bits = 0)
Broadcast Address: 192.168.1.255 (all host bits = 1)
Usable Host Range: 192.168.1.1 - 192.168.1.254
Number of Usable Hosts: 2⁸ - 2 = 254

Dividing /24 into Subnets

Divide 192.168.1.0/24 into 4 equal subnets (/26):

Subnet	Network Address	Broadcast	Host Range	Hosts
1	192.168.1.0/26	192.168.1.63	192.168.1.1 - .62	62
2	192.168.1.64/26	192.168.1.127	192.168.1.65 - .126	62
3	192.168.1.128/26	192.168.1.191	192.168.1.129 - .190	62
4	192.168.1.192/26	192.168.1.255	192.168.1.193 - .254	62

VLSM (Variable Length Subnet Masking)

VLSM allows using different subnet masks for different subnets within the same network, enabling more efficient use of IP addresses. Instead of dividing a network into equal-sized subnets, you allocate exactly the right size for each subnet.

Example: You have 192.168.1.0/24 and need:

Subnet A: 100 hosts -> use /25 (126 hosts) -> 192.168.1.0/25
Subnet B: 50 hosts -> use /26 (62 hosts) -> 192.168.1.128/26
Subnet C: 25 hosts -> use /27 (30 hosts) -> 192.168.1.192/27
Subnet D: 10 hosts -> use /28 (14 hosts) -> 192.168.1.224/28

Key Takeaways

Subnetting divides a large network into smaller, more manageable sub-networks.
A subnet mask determines which part of an IP address is the network and which is the host.
CIDR notation (e.g., 192.168.1.0/24) specifies the network address and prefix length.
/24 means 24 bits for network, 8 bits for hosts - allowing 254 usable host addresses.
The first address in a subnet is the network address; the last is the broadcast address.
Subnetting improves security, reduces broadcast traffic, and makes IP address management easier.