Subnetting is the process of breaking down an IP network into smaller subnets called "subnets." Each subnet is a non-physical description (or ID) for a physical sub-network (typically the network switches from hosts containing one router to a multi-router network).
Why should I do subnetting? There are several reasons why we need to do subnetting, including the following :
- To streamline the allocation of IP addresses in a network in order to maximize the use of IP addresses.
- Overcoming the problem of differences in hardware and physical media used in a network, because IP routers can only integrate multiple networks with different physical media if each network has a unique network address.
- Improve security and reduce congestion due to too many hosts on a network.
Subnetting calculations can be done in two ways, namely binary which is relatively slow and the specific method is faster. Writing the IP address is generally 192.168.1.2. However, sometimes it is written as 192.168.1.2/24. The explanation is that the IP address is 192.168.1.2 with a subnet mask of 255.255.255.0. Why is it like? The meaning of / 24 is taken from the calculation that the 24 bit subnet mask is covered with binary 1. Or in other words, the subnet mask is: 11111111.11111111.11111111.00000000 (255.255.255.0). This concept is called CIDR (Classless Inter-Domain Routing) which was first introduced in 1992 by IEFT. Virtually all subnetting questions will revolve around four issues: Number of Subnets, Number of Hosts per Subnet, Subnet Block, and Host-Broadcast Address.
1. An example of a Subnetting case that occurs with a NETWORK ADDRESS 10.0.0.0/16.
SUBNETTING TO IP ADDRESS CLASS A
Class A in octets 2, 3 and 4 (last 3 octets). Then the subnet masks that can be used for class A subnetting are all subnet masks from CIDR / 8 to / 30.
Analysis:
10.0.0.0 means class A, with the Subnet Mask / 16 means 11111111.11111111.00000000.00000000 (255.255.0.0).
Calculation:
Number of Subnets = 28 = 256 subnets
The number of hosts per subnet = 216 - 2 = 65534 hosts
Subnet block = 256 - 255 = 1. So the complete subnet: 0,1,2,3,4, etc.
2. Examples of cases of Subnetting that occur with a NETWORK ADDRESS 172.16.0.0/18 and 172.16.0.0/25.
SUBNETTING ON IP ADDRESS CLASS B
Based on the subnet block. CIDR / 17 to / 24 are exactly the same as Class C subnetting, only we enter the subnet block directly into the third octet, not like Class C which is "played" in the fourth octet. While the CIDR / 25 to / 30 (multiples) of the subnet block we "play" in the fourth octet, but after the third octet finishes, it goes forward (coeunter) from 0, 1, 2, 3, etc.
>> Example of network address 172.16.0.0/18
Analysis:
172.16.0.0 means class B, with the Subnet Mask / 18 means 11111111.11111111.11000000.00000000 (255.255.192.0).
Calculation:
Number of Subnets = 2x, where x is the number of binaries 1 in the last 2 octets. So the number of subnets is 22 = 4 subnets
Number of Hosts per Subnet = 2y - 2, where y is the reciprocal of x, i.e. the number of binaries 0 in the last 2 octets. So the number of hosts per subnet is 214 - 2 = 16,382 hosts
Subnet block = 256 - 192 = 64.The next subnet is 64 + 64 = 128, and 128 + 64 = 192. So the complete subnets are 0, 64, 128, 192.
>> Example of network address 172.16.0.0/25.
Analysis:
172.16.0.0 means class B, with the Subnet Mask / 25 means 11111111.11111111.11111111.10000000 (255.255.255.128).
Calculation:
Number of Subnets = 29 = 512 subnets
The number of hosts per subnet = 27 - 2 = 126 hosts
Subnet block = 256 - 128 = 128. So the complete is (0, 128)
3. Examples of cases of Subnetting that occur with a NETWORK ADDRESS 192.168.1.0/26
SUBNETTING ON IP ADDRESS CLASS C
Analysis:
192.168.1.0 means class C with Subnet Mask / 26 means 11111111.11111111.11111111.11000000 (255.255.255.192).
Calculation:
Number of Subnets = 2x, where x is the number of binaries 1 in the last octet of the subnet mask (the last 2 octets for class B, and the last 3 octets for class A). So the number of subnets is 22 = 4 subnets
Number of Hosts per Subnet = 2y - 2, where y is the reciprocal of x, which is the number of binaries 0 in the last octet of the subnet. So the number of hosts per subnet is 26 - 2 = 62 hosts
Subnet block = 256 - 192 (the last octet value of the subnet mask) = 64.The next subnet is 64 + 64 = 128, and 128 + 64 = 192. So the complete subnets are 0, 64, 128, 192.
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