The /18 subnet mask is a subnet mask with a dotted decimal notation of 255.255.192.0 and a wildcard mask of 0.0.63.255.
The 18 subnet mask divides larger networks like class A and class B networks into smaller networks (subnetworks). Each of these subnetworks has a range of 16384 IP addresses. The first IP address in this range is the network address, and the last IP address in this range is the broadcast IP address. Therefore, there are 16382 usable IP addresses in a /18 subnet mask.
A /18 subnet mask can be subneted further with a /19 subnet mask, /20 subnet mask and other higher prefix length we covered earlier.
Subnetting with a /18 Subnet Mask (Example)
When subnetting a network with a /18 subnet mask, you have a total of 16384 IP addresses in each subnet. Let’s consider subnetting the network 192.168.0.0/16 (a class B network) with a /18 subnet mask:
First Subnet:
- Network Address: 192.168.0.0
- Usable IP Range: 192.168.0.1 to 192.168.63.254
- Broadcast IP Address: 192.168.63.255
Second Subnet:
- Network Address: 192.168.64.0
- Usable IP Range: 192.168.64.1 to 192.168.127.254
- Broadcast IP Address: 192.168.127.255
Here is a table of a network subnetted with /18 subnet mask;
| Subnet | Network Address | IP Range | Broadcast IP Address |
|---|---|---|---|
| 1 | 192.168.0.0 | 192.168.0.1 to 192.168.63.254 | 192.168.63.255 |
| 2 | 192.168.64.0 | 192.168.64.1 to 192.168.127.254 | 192.168.127.255 |
| 3 | 192.168.128.0 | 192.168.128.1 to 192.168.191.254 | 192.168.191.255 |
| 4 | 192.168.192.0 | 192.168.192.1 to 192.168.255.254 | 192.168.255.255 |
/18 Subnet Mask IP range
For a /18 subnet mask, there is a range of 16384 IP addresses. The first IP address in this range serves as the network ID, and the last IP address functions as the broadcast IP address. Hence, there are precisely 16382 usable IP addresses in a /18 subnet mask.
/18 Subnet Mask Dotted Decimal Notation
The dotted decimal notation for the 18 subnet mask is 255.255.192.0. This value is gotten by setting the first 18 bits of the 32-bit IPv4 address to 1s, setting the remaining 14 bits to 0s, and then converting from binary to decimal.
Subnetting Class B Network with /18 Subnet Mask
To subnet a class B network with a /18 subnet mask, we need to borrow 2 bits from the host bits, resulting in the binary representation 11111111.11111111.11000000.00000000. Remember, the original binary representation for the class B subnet mask is 111111111.11111111.00000000.00000000.
Number of Subnets
The formula for calculating the number of subnets remains 2^(number of borrowed bits). In this case, it yields 2^2 = 4 subnets.
However, adhering to RFC guidelines, two subnets are reserved for specific functions. Therefore, the usable number of subnets is 4 – 2 = 2.
Subnetting Class A Network with /18 Subnet Mask
To subnet a class A network with a /18 subnet mask, which has a binary representation of 11111111.00000000.00000000.00000000, 10 bits are borrowed from the host bits and added to the network bits. This results in the binary representation 11111111.11111111.11000000.00000000.
Number of Subnets
The formula for calculating the number of subnets is 2^(number of borrowed bits). In this case, it results in 2^10 = 1024 subnets.
However, according to RFC guidelines, two subnets are reserved for specific functions. Therefore, the usable number of subnets becomes 1024 – 2 =1022 usable subnets.
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