The /21 subnet mask is a subnet mask with dotted decimal notation of 255.255.248.0 and a wildcard mask of 0.0.7.255.
21 subnet masks divide larger networks, like class A networks and class B networks, into smaller networks (subnetworks). Each of the subnetworks has a range of 2048 IP addresses. The first IP address in this range is the network ID, and the last IP address in this range is the broadcast address. Hence, there are 2046 usable IP addresses in a /21 subnet mask.
A /21 subnet mask can be subnetted with a /22 subnet mask, a /23 subnet mask, a /24 subnet mask, and other higher subnet masks that we discussed earlier.
Subnetting a Network with /21 Subnet Mask
When a large network is subnetted with a /21 subnet mask, it produces a smaller network (subnetworks) with a 2048 range of IP addresses.
For example, if we subnet the network; 192.168.0.0/16 (a class B network) with a /21 subnet mask, the subnetting would be as follows:
Network Address for the First Subnet: 192.168.0.0
Usable IP Range for the First Subnet: 192.168.0.1 to 192.168.7.254
Broadcast IP Address for the First Subnet: 192.168.7.255
Network Address for the Second Subnet: 192.168.8.0
Usable IP Range for the Second Subnet: 192.168.8.1 to 192.168.15.254
Broadcast IP Address for the Second Subnet: 192.168.15.255
Here is a table of class B network subnetted with a /21 subnet mask;
| Subnet | Network Address | IP Range | Broadcast Address |
|---|---|---|---|
| 1 | 192.168.0.0 | 192.168.0.1 to 192.168.7.254 | 192.168.7.255 |
| 2 | 192.168.8.0 | 192.168.8.1 to 192.168.15.254 | 192.168.15.255 |
| 3 | 192.168.16.0 | 192.168.16.1 to 192.168.23.254 | 192.168.23.255 |
| 4 | 192.168.24.0 | 192.168.24.1 to 192.168.31.254 | 192.168.31.255 |
| 5 | 192.168.32.0 | 192.168.32.1 to 192.168.39.254 | 192.168.39.255 |
| 6 | 192.168.40.0 | 192.168.40.1 to 192.168.47.254 | 192.168.47.255 |
| 7 | 192.168.48.0 | 192.168.48.1 to 192.168.55.254 | 192.168.55.255 |
| 8 | 192.168.56.0 | 192.168.56.1 to 192.168.63.254 | 192.168.63.255 |
| 9 | 192.168.64.0 | 192.168.64.1 to 192.168.71.254 | 192.168.71.255 |
| 10 | 192.168.72.0 | 192.168.72.1 to 192.168.79.254 | 192.168.79.255 |
| 11 | 192.168.80.0 | 192.168.80.1 to 192.168.87.254 | 192.168.87.255 |
| 12 | 192.168.88.0 | 192.168.88.1 to 192.168.95.254 | 192.168.95.255 |
| 13 | 192.168.96.0 | 192.168.96.1 to 192.168.103.254 | 192.168.103.255 |
| 14 | 192.168.104.0 | 192.168.104.1 to 192.168.111.254 | 192.168.111.255 |
| 15 | 192.168.112.0 | 192.168.112.1 to 192.168.119.254 | 192.168.119.255 |
| 16 | 192.168.120.0 | 192.168.120.1 to 192.168.127.254 | 192.168.127.255 |
| 17 | 192.168.128.0 | 192.168.128.1 to 192.168.135.254 | 192.168.135.255 |
| 18 | 192.168.136.0 | 192.168.136.1 to 192.168.143.254 | 192.168.143.255 |
| 19 | 192.168.144.0 | 192.168.144.1 to 192.168.151.254 | 192.168.151.255 |
| 20 | 192.168.152.0 | 192.168.152.1 to 192.168.159.254 | 192.168.159.255 |
| 21 | 192.168.160.0 | 192.168.160.1 to 192.168.167.254 | 192.168.167.255 |
| 22 | 192.168.168.0 | 192.168.168.1 to 192.168.175.254 | 192.168.175.255 |
| 23 | 192.168.176.0 | 192.168.176.1 to 192.168.183.254 | 192.168.183.255 |
| 24 | 192.168.184.0 | 192.168.184.1 to 192.168.191.254 | 192.168.191.255 |
| 25 | 192.168.192.0 | 192.168.192.1 to 192.168.199.254 | 192.168.199.255 |
| 26 | 192.168.200.0 | 192.168.200.1 to 192.168.207.254 | 192.168.207.255 |
| 27 | 192.168.208.0 | 192.168.208.1 to 192.168.215.254 | 192.168.215.255 |
| 28 | 192.168.216.0 | 192.168.216.1 to 192.168.223.254 | 192.168.223.255 |
| 29 | 192.168.224.0 | 192.168.224.1 to 192.168.231.254 | 192.168.231.255 |
| 30 | 192.168.232.0 | 192.168.232.1 to 192.168.239.254 | 192.168.239.255 |
| 31 | 192.168.240.0 | 192.168.240.1 to 192.168.247.254 | 192.168.247.255 |
| 32 | 192.168.248.0 | 192.168.248.1 to 192.168.255.254 | 192.168.255.255 |
/21 Subnet Mask Usable IP Address
There are 2048 ranges of IP addresses in a /21 subnet mask. The first IP address in this range is the network ID, and the last in this range is the broadcast IP address. Hence, there are 2046 usable IP addresses in a /21 subnet mask.
/21 Subnet Mask Dotted Decimal Notation
For a /21 subnet mask, the dotted decimal notation is 255.255.248.0. This value is obtained by setting the first 21 bits of the 32-bit IPv4 address to ‘1s’, setting the remaining 11 bits to ‘0s’, and then converting from binary to decimal.
Subnetting Class B Network with /21 Subnet Mask
To subnet a class B network with a /21 subnet mask, we need to borrow 5 bits from the host bits to get the binary representation of a /21 subnet mask, which is 111111111.11111111.11111000.00000000 from the binary representation of the subnet mask of class B networks, which is 111111111.11111111.00000000.00000000.
When subnetting a class B network with a /21 subnet mask, you are borrowing 5 bits for subnetting. The formula for calculating the number of subnets is 2^(number of borrowed bits).
In this case, it would be 2^5 = 32 subnets.
However, as per RFC guidelines, two subnets are reserved for specific functions, similar to the scenario with a /22 subnet mask
So, when subnetting a class B network with a /21 subnet mask, you will obtain 30 usable subnets.
Subnetting Class A Network with /21 Subnet Mask
To subnet a class A network with a /21 subnet mask, we need to borrow 13 bits from the host bits to get the binary representation of a /21 subnet mask, which is 11111111.11111111.11100000.00000000. This is derived from the binary representation of the subnet mask of class A networks, which is 11111111.00000000.00000000.00000000.
When subnetting a class A network with a /21 subnet mask, you are borrowing 13 bits for subnetting. The formula for calculating the number of subnets is 2^(number of borrowed bits).
In this case, it would be 2^13 = 8192 subnets.
However, as per RFC guidelines, two subnets are reserved for specific functions, similar to the scenario with a /22 subnet mask. Therefore, the usable number of subnets will be 8192 – 2 = 8190.
How many hosts are in 21?
There are a total of 2048 ranges of IP addresses in the /21 subnet. The first IP address in this range is the network ID, and the last IP address is the broadcast address. Hence, there is a 2046 usable IP address in a /22 subnet mask.
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