The /25 subnet mask divides a large network into multiple smaller subnetworks. Each of the subnetworks comprises 128 ranges of IP addresses. The first IP address in this range is reserved as the network address, while the last IP address in this range is reserved as the broadcast IP address. Hence, there are 126 usable IP addresses in a /25 subnet mask.
For instance, if we subnet the network 192.168.2.0/24 with a /25 subnet mask, the first subnet will have the network address 192.168.2.0, a usable IP range from 192.168.2.1 to 192.168.2.126, and the broadcast address 192.168.2.127. The second subnet will have a network address of 192.168.2.128, a usable IP range of 192.168.2.129 to 192.168.254, and a broadcast address of 192.168.2255. Hence, the table of IPs of a network subnetted with a /25 subnet mask is shown below:
| Network Address | IP address range | Broadcast IP address |
|---|---|---|
| .0 | .1 - .126 | .127 |
| .128 | .129 - .254 | .255 |
/25 Subnet Mask Usable IP Address
In /25 subnet mask, there are 128 ranges of IP addresses; the first one in the range is reserved for a network address, and the last IP address in this range is reserved as a broadcast IP address. Hence, there are 126 usable IP addresses in the /25 prefix length.
/25 Subnet Mask Dotted Decimal Notation
The dotted decimal notation for a /25 subnet mask is 255.255.255.128. This value is gotten by setting the first 25 bits of the 32-bit IPv4 address to ‘1s’, setting the remaining 7 bits to ‘0s’, and converting from binary to decimals.
Subnetting Class C Network with /25 Subnet Mask
To subnet class C network from the /25 subnet mask, one bit is borrowed from the host bits and added to the network bits;
Hence, the number of subnets obtainable when subnetting a class network with a /25 prefix length is 2. This is gotten from the math: 2^1 = 2. where 1 is the number of borrowed bits.
Subnetting Class B Network with /25 Subnet Mask
To subnet a Class B network using the /25 prefix length, 9 bits need to be borrowed from the host bit and added to the network bits.
Hence, the number of subnetworks obtainable when a class B network is subneted with a /25 subnet mask is 512. This is gotten from the math: 2^9 = 512. Where 9 is the number of borrowed bits.
Subnetting Class A Network with /25 Subnet Mask
To subnet a class A network with a /25 prefix length, 17 bits need to be borrowed from the host bits and added to the network bits. The binary notation for the Class A network, which has an /8 subnet mask, is 111111111.00000000.00000000. Hence, we borrow 17 bits from the host bits and add them to the network bits to obtain 11111111.11111111.10000000, which is now the binary notation for the /25 subnet mask.
So, the number of subnetworks obtainable when we subnet a class A network with a /25 prefix length is 131072. This is gotten by the math: 2^17 = 131072. Where 17 is the number of borrowed bits.
How Many host in /25
There are 128 ranges of IP addresses in a /25 prefix length; the first IP address in this range is the network ID, and the last IP address in this range is the broadcasd address. Hence, there are 126 IP addresses available to be assigned to a host in a /25 subnet mask.
How Many Subnets in /25
The number of subnetworks in a /25 subnet mask depends on the class of the network being subneted.
Class C networks yield 2 subnetworks; class B networks yield 512 subnetworks; and class A networks yield 131072 subnetworks when subneted with a /25 subnet mask.
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