What is subnet mask 27 and how to calculate the actual subnet mask that is 255.255.255.224
Subnet mask 27 can be obtained by subnetting the other default subnets. When subnetting the default class c subnet, we have to change 3 host bits to network bits to create more networks and reduce the number of hosts.
Subnet mask 27 or 255.255.255.224 in binary is 11111111.11111111.11111111.11100000.
If we change the first three host bits to network bits in the last octet, the total comes up as 224 so after subnetting the 255.255.255.0, the subnet mask becomes 255.255.255.224.
Using this subnet mask will give us 8 subnets when subnetting the class c subnet. We can use the following trick to find out the number of networks available while using this subnet mask.
In the last octet, we have three network bits so we will multiply 2x2x2 and we receive the number 8 now we have to find out the network ranges available to use in our network.
In this example, we are using the default class c subnet which is 192.168.1.0/24; this network will be subnetted to create more networks and ranges.
Following are the 8 network ranges created.
192.168.1.0 192.168.1.31
192.168.1.32 192.168.1.63
192.168.1.64 192.168.1.95
192.168.1.96 192.168.1.127
192.168.1.128 192.168.1.159
192.168.1.160 192.168.1.191
192.168.1.192 192.168.1.223
192.168.1.224 192.168.1.255
In one network range, we have 32 hosts available however we can only assign 30 to the devices as two IPs are reserved for subnet ID and broadcast address for particular subnet.
How to assign /27 subnet mask to the devices
While assigning the IP addresses, we have to make sure that we are using valid IP addresses according to the subnet ranges and subnet mask should be changed from default to 255.255.255.224.
In the below image, fix the IP address used. Take help from the above-shared subnet ranges to find out the issues with the IP addressing.
The network diagram is using /27 subnet.
There are two issues with the IP addresses used.
- In network 1, IP address 192.168.1.1 and 192.168.1.33 is on a different subnet so we have to use the IP addresses either from 192.168.1.0/27 or 192.168.1.32/27 subnet.
- In network 2, the IP address assigned to the router is the network ID so we cannot assign this IP address, we have to use any other IP address from this range.
