The Subnet Mask and Slash Notation

An IP address consists of two addresses: the network address and the host address. A subnet mask separates the network address from the host address in the IP address. An IP address is always written and used with the subnet mask. This tutorial explains the fundamentals of a subnet mask.

A subnet mask defines the boundary between the network address and the host address in the IP address. A network address is the group address. It is used to create a group of IP addresses. A group of IP addresses is known as an IP subnet. A host address is an individual address. In an IP subnet, all addresses have the same network address and different host addresses.

The following image shows four IP subnets with six host addresses.

In binary, an IP address is 32 bits long. It divides these bits into four sections. In each, it keeps 8 bits or 1 byte (8 bits = 1 byte). It uses dots to separate them. From 8 binary bits, the maximum number you can make in decimal is 256. IP addresses start from 0. Hence, the maximum number you can have in each section in decimal is 255 (256 - 1).

The subnet mask separates network and host bits in an IP address. For 32 IP bits, 32 subnet mask bits are used. Subnet mask bits and IP address bits have one-to-one relations. The first subnet mask bit relates to the first IP bit, the second subnet mask bit to the second IP bit, and so on til the last bit.

A subnet mask bit shows the type of the related IP bit. If its value is 1, the IP bit will be a network bit. If its value is 0, the IP bit will be a host bit.

There are five IP classes : A, B, C, D, and E. Only classes A, B, and C are available for network addressing. You cannot use classes D and E for network addressing. Network services use class D for multicast addressing. Class E is reserved for research.

Default subnet mask

IP addresses in classes D and E have all 1s in the network bits. They do not have host bits. Since they do not have host bits, and a subnet mask is used to differentiate between host and network bits, they do not need a subnet mask.

Only IP addresses in classes A, B, and C have host bits and need a subnet mask. These classes have a default subnet mask. The following table lists default subnet masks.

Because of the default subnet masks, the first 8, 16, and 24 bits in classes A, B, and C are always network bits.

Slash notation

Slash notation is a shorter way to write subnet masks. In this notation, instead of writing the full subnet mask, you use only the number of bits having the value 1 after the slash with the IP address.

For example, the default subnet mask of class A is 255.0.0.0. In binary, it is 11111111.00000000.00000000.00000000. It has 8 network bits set to 1. In slash notation, you will write this subnet mask as /8.

The default subnet mask for a class B network has 16 network bits. In slash notation, it will be /16. The default subnet mask for a class C network has 24 network bits. In slash notation, we will write it as /24.

The following table lists some examples of IP addresses and their subnet mask in all three notations.

Bits in the subnet mask

Bits in the subnet mask always follow a pattern. They are written from left to right. Network bits are always written before the host bits. You cannot write a host bit between network bits.

Subnetting

Subnetting is the process of dividing an IP subnet into smaller subnets. To break an IP subnet, you convert the host bits into network bits. The conversion process starts from the leftmost host bit.

When converting bits, you cannot skip any host bit. For example, if you want to split a subnet into two subnets, set the leftmost host bit to 1 to convert it into a network bit. If you want to break it into four subnets, set the two leftmost host bits to 1 to treat them as network bits.

Key points:-

• A subnet mask is a 32-bit binary address.
• IP addresses use subnet masks to separate network bits and host bits.
• Subnet mask bits and IP address bits are in a one-to-one relationship.
• The first subnet mask bit corresponds to the first IP bit, the second subnet mask bit corresponds to the second IP bit, and so on until the last bit.
• If the subnet mask bit is 1, the corresponding IP address bit is part of the network address.
• If the subnet mask bit is 0, the corresponding IP address bit is part of the network address.
• Subnet mask bits are written from left to right.
• In a subnet, network bits are always written before the host bits.
• Subnetting is the process of dividing a subnet into multiple subnets.
• In subnetting, host bits are converted into network bits.
• Subnetting always starts from the leftmost host bit and goes to the rightmost bit without skipping any bit.

Conclusion

An IP address is always written with a subnet mask. It contains a network address and a host address. The subnet mask defines the network and host addresses within the IP address. It also allows you to extend the default network address of the IP address. Extending the default network address is called subnetting. Understanding subnet masks and subnetting is essential for effective network management. Mastery of these concepts enables you to efficiently allocate IP addresses, enhance network security, and ensure optimal network performance.