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Subnet Mask Cheat Sheet: A Guide to Subnetting

A subnet mask is a 32-bit number that defines how an IP address is divided into network and host parts. Subnetting is the process of creating smaller networks from a larger network by applying different subnet masks. Subnetting can help improve network performance, security, and management.

This subnet mask cheat sheet provides a quick reference for common subnet masks and their corresponding CIDR notation, wildcard mask, number of hosts per subnet, and binary representation. You can use this cheat sheet to understand and calculate subnets for your network.

What is a Subnet Mask

A subnet mask is a 32-bit number that works as a filter for an IP address. It tells devices which part of the IP address is the network identifier and which part is the host identifier. For example, the subnet mask 255.255.255.0 means that the first 24 bits of the IP address are the network part and the last 8 bits are the host part.

Subnet masks are usually written in dotted decimal notation, which is the same format as IP addresses. Each octet (8-bit block) in the subnet mask can have a value from 0 to 255. However, not all combinations of values are valid. A valid subnet mask must have consecutive 1s followed by consecutive 0s in binary form. For example, 255.255.255.0 is a valid subnet mask because it has 24 1s followed by 8 0s in binary (11111111.11111111.11111111.00000000), but 255.255.255.128 is not a valid subnet mask because it has a 0 between two 1s in binary (11111111.11111111.11111111.10000000).

What is CIDR Notation

CIDR notation is a shorthand way of writing subnet masks. It stands for Classless Inter-Domain Routing and it uses a slash (/) followed by a number to indicate how many bits are used for the network part of the IP address. For example, /24 means that 24 bits are used for the network part and 8 bits are used for the host part.

CIDR notation is useful because it saves space and makes it easier to see how many subnets and hosts are possible with a given subnet mask. The table below shows some common subnet masks and their equivalent CIDR notation.

Subnet MaskCIDR Notation

255.255.255.0/24

255.255.255.128/25

255.255.255.192/26

255.255.255.224/27

255.255.255.240/28

255.255.255.248/29

255.255.255.252/30

255.255.254.0/23

255.255.252.0/22

255.255.248.0/21

255.255.240.0/20

255.255.224.0/19

255.255.192.

What is a Wildcard Mask

A wildcard mask is another way of representing a subnet mask. It is the inverse of the subnet mask, meaning that any bit that is 0 in the subnet mask is 1 in the wildcard mask, and vice versa. For example, the wildcard mask for 255.255.255.0 is 0.0.0.255.

Wildcard masks are mainly used in access control lists (ACLs) to specify which IP addresses are allowed or denied access to a network device or service. For example, an ACL entry that uses the wildcard mask 0.0.0.255 can match any IP address that has the same first three octets as the specified IP address.

How to Subnet

Subnetting is the process of dividing a network into smaller subnetworks by applying different subnet masks. Subnetting can help optimize network performance, security, and management by reducing broadcast traffic, isolating network problems, and creating logical network segments.

To subnet a network, you need to follow these steps:

Determine how many subnets and hosts per subnet you need based on your network requirements.

Choose a suitable subnet mask that can accommodate your subnets and hosts. You can use the table above or the formulas below to calculate the number of subnets and hosts per subnet for a given subnet mask.

Number of subnets = 2 where n is the number of bits borrowed to make the subnet mask

Number of hosts = 2 â 2 where n is the number of bits in your subnet mask

Write down the network address and the subnet mask in binary form.

Draw a vertical line between the network bits and the host bits in both the network address and the subnet mask.

To find the first subnet, copy the network bits from the network address and add a 0 for each host bit.

To find the second subnet, copy the network bits from the network address and add a 1 for the first host bit and 0s for the rest of the host bits.

To find the subsequent subnets, increment the host bits by 1 in binary form until you reach the maximum number of subnets.

To find the broadcast address for each subnet, copy the network bits from the subnet address and add a 1 for each host bit.

To find the usable host range for each subnet, add 1 to the subnet address for the first host and subtract 1 from the broadcast address for the last host.

Convert all binary addresses back to decimal form.

For example, suppose you have a network with an IP address of 192.168.1.0/24 and you want to create four subnets with equal number of hosts per subnet. Here are the steps to subnet this network:

You need four subnets and at least 62 hosts per subnet.

You can choose a /26 subnet mask (255.255.255.192) which can create four subnets with 62 hosts per subnet.

The network address and the subnet mask in binary form are:

11000000.10101000.00000001.00000000 (192.168.1.0)

11111111.11111111.11111111.11000000 (255.255.255.192)

The vertical line between the network bits and host bits is:

11000000.10101000.00000001.00000000 (192.168.1.0)

11111111.11111111.11111111.11000000 (255.255.255.192)

The first subnet is:

11000000.10101000.00000001.00000000 (192.168.1.0)

The second subnet is:

11000000.10101000.00000001.01000000 (192.168.1 aa16f39245