IP Address Subnetting


IP Subnetting and Subnetting Examples

As we talked about before, there are two parts in an IP address. One for them is Network part and the another is Host part. With IP Subnetting, we are adding one more part. This is “Subnet Part”. From the Host part, we borrow some bits and we will use this part for Subnet.


As a basic definion, Subnetting is dividing the network into smaller network groups and by doing this, using the IP Address Block more efficient.

For Subnetting, Subnet Masks are used. Subnets masks are 32 bit addresses like IP Addresses. Subnet Masks are used with IP Addresses. The 1s represents the network parts, and 0s represents the host parts.

We can show Subnet Masks with four octets like IP addresses ( or we can show it like /X . Here, for the Subnet Mask, we can use /24. This means that the first 24 bit is full of 1s and it is network part.

In Subnetting, there are two important terms. These are :

CIDR (Classless Inter Domain Routing)
VLSM (Variable Lenght Subnet Mask)

CIDR (Classless Inter Domain Routing) is the term that is used for using IP addresses independent from their traditional IP Classes. In other words, CIDR is using IP addresses without classes.

VLSM (Variable Lenght Subnet Mask) is the term that is used for using different Subnet Mask for different sun networks. In aother words, it is the mechanism that allows different Subnet Masks and provide division of a network into sub networks. It is like Subnet of subnets.

CIDR is used on the addresses that will advertise to the internet. So, it is used in the Internet Service Provider part.VLSM is used in a company or in smaller networks to use IP address spaces ideally.
Subnetting is one of the important lessons of Networking. So, we will show this with more examples.

Special Subnets

In Subnetting some Subnet Masks are used specifically sometimes. These are /24, /30, /31/ and /32.

/24 is the Subnet Mask that is usually used in the local networks by default.
/32 is the Subnet Mask used generally on Loopback and System interfaces.
/31 is the Subnet Mask used on point-to-point links.
/30 is also widely used in Service Provider Networks for point-to-point connections.

Loopback Interface is the “virtual” interfaces. There can be many Loopback interfaces in a Router. Loopback Interfaces are used for its “always up and never physically down” characteristics generally. We give these Loopback Interfaces a /32 Loopback IP address.

There is also a System Address that is used on Alcatel-Lucent Service Routers. This is a specific loopback address that provide to reach the router’s itself. This address is very important for ALU routers.It is used in many protocol configurations. System addresses are /32 IP addresses.

IP Subnetting Example 1

In this first example we will use, /24 Address.Let’s determine the network and host part of this address. This is the first example, so we are starting with an easy example.

IP Address :
Subnet Mask :

For this example, firstly we will convert this decimal numbers to the binary equals. As you can see below, the 1s in the Subnet Mask will show the number of bits that network part has. And the 0s will show the host part bits.

IP Address : 11000000. 10101000.00000101.01010101
Subnet Mask : 11111111. 11111111. 11111111.00000000

So, here, the first 24 bits (First 3 octets) are network bits and the last 8 bits (Last octet) are the host bits.

For this IP and Subnet Mask, to determine the Network Address of this IP address, we will use “AND” operation between IP Address and Subnet Mask in binary mode.

IP Add: 11000000. 10101000.00000101.01010101
SubM : 11111111. 11111111. 11111111.00000000
AND : 11000000. 10101000.00000101.00000000

When we use AND operation with this binary numbers, as you can see, the last octet will be multiple with zero (AND is Multiplication). So the result of this multiplication will be Here, the first three octets will be same as IP address and the last octet will be full of 0s.

For this example our broadcast address will be AS you can see, all the host bits are full of 1s for broadcast address. The other addresses in the middle through to are host addresses.

IP Subnetting Example 2

In this second example, we will do a little more complex example. This time our IP address will be

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Lesson tags: ip addressing, IPv4
Back to: JNCIA Junos > IP Addressing

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