What Is a Custom Subnet Mask


What is Subnet Mask?

A subnet mask is a 32-bit number created by setting host bits to all 0s and setting network bits to all 1s. In this way, the subnet mask separates the IP address into the network and host addresses.

The “255” address is always assigned to a broadcast address, and the “0” address is always assigned to a network address. Neither can be assigned to hosts, as they are reserved for these special purposes.

The IP address, subnet mask and gateway or router comprise an underlying structure—the Internet Protocol—that most networks use to facilitate inter-device communication.

When organizations need additional subnetworking, subnetting divides the host element of the IP address further into a subnet. The goal of subnet masks are simply to enable the subnetting process. The phrase “mask” is applied because the subnet mask essentially uses its own 32-bit number to mask the IP address.

A subnetwork or subnet is a logical subdivision of an IP network. The practice of dividing a network into two or more networks is called subnetting. Computers that belong to the same subnet are addressed with an identical most-significant bit-group in their IP addresses

IP Address and Subnet Mask

A 32-bit IP address uniquely identifies a single device on an IP network. The 32 binary bits are divided into the host and network sections by the subnet mask but they are also broken into four 8-bit octets.

Because binary is challenging, we convert each octet so they are expressed in dot decimal. This results in the characteristic dotted decimal format for IP addresses—for example, The range of values in decimal is 0 to 255 because that represents 00000000 to 11111111 in binary.

What are the 3 classes of subnet mask?

The three default subnet masks are 255.0. 0.0 for Class A, 255.255. 0.0 for class B, and 255.255. 255.0 for Class C.


  • Subnetting allows you to divide one physical network whether a class A, B, or C into several smaller logical subnetworks.
  • The main idea of subnetting is borrowing bits from the host IP address and using them to create one or more networks within the network.
  • Furthermore, It permits using two or more local area networks (LANs) technology in the same network such as Ethernet and Toking ring.

Advantages of using subnetting:

  • It is useful to control and to reduce the network traffic by limiting number of broadcasts.
  • It is allowed any organization to subnet its network without needed to have a new network IP through an internet service provider (ISP).
  • Subnetting was so helpful to solve the problem of lacking IP addresses on the Internet.
  • Allowing to use two or more LAN technologies together in the same network.
  • Subnets also helpful to minimize the size of the routing tables on the internet since additional network numbers will not being added to the table.

a. When you want to isolate segments for security reasons such as accounting and sales segment.

b. When you want to isolate bad segments such as domination hosts which use most of the LAN Bandwidth.

Disadvantage Advantages of using subnetting:

  • Subnetting decreases the total number of IP addresses in the network but may need buying additional hardware such as a router. So, it may cost lots of money.
  • It cannot correct the lack of efficiency because companies still assign address block regarding to classes.


Custom Subnet Mask

Subnetting is accomplished by using bits from the host address space for the network address space.■

The custom subnet mask, also called a variable length subnet mask, is used to identify the bits used for a network address versus the bits used for a host address.■

Custom subnet masks are used when subnetting or supernetting.

To determine the appropriate custom subnet mask, typically referred to simply as subnet mask, for a network, you must follow these steps:1.

Determine the number of host bits to be used for subnetting.a.

Determine the maximum number of subnets required including consideration for future anticipated growth.b.

Determine how many host bits are required to create the number of subnets.c.

Add together the values of the left-most bits from the octets, yielding the highest network ID.2.

Determine the new subnetted network IDs.a.

List all the possible binary combinations of the bits taken from the host address space.b.

Calculate the incremental value to each subnet and add to the network address.3.

Determine the IP addresses for each new subnet.a.

Start with counting out the default class network ID bits.b.

Add the bits that were borrowed from the host ID to the default network ID.4.

Determine the appropriate subnet mask.a.

Use bitwise ANDing to compare the bits of the IP address and the subnet mask.b.

The result of the comparison is the network ID.

How do I find my custom subnet mask?

Convert Subnet Mask To Dotted Decimal Notation: We take each of the octets in the subnet mask and convert it to decimal. The result is our custom subnet mask in the form we usually see it: 255.255. 255.224. Express Subnet Mask In “Slash Notation”: Alternately, we can express the subnet mask in “slash notation”.


Author: refuge_2020

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