/21 Subnet Mask | /21 IP Range

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.

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Everything About /20 Subnet Mask/30 Subnet Mask
/21 Subnet Mask | /21 IP Range/31 Subnet Mask
/22 Subnet Mask/32 Subnet Mask
/23 Subnet Mask/28 Subnet Mask
/29 Subnet Mask

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

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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;

SubnetNetwork AddressIP RangeBroadcast Address
1192.168.0.0192.168.0.1 to 192.168.7.254192.168.7.255
2192.168.8.0192.168.8.1 to 192.168.15.254192.168.15.255
3192.168.16.0192.168.16.1 to 192.168.23.254192.168.23.255
4192.168.24.0192.168.24.1 to 192.168.31.254192.168.31.255
5192.168.32.0192.168.32.1 to 192.168.39.254192.168.39.255
6192.168.40.0192.168.40.1 to 192.168.47.254192.168.47.255
7192.168.48.0192.168.48.1 to 192.168.55.254192.168.55.255
8192.168.56.0192.168.56.1 to 192.168.63.254192.168.63.255
9192.168.64.0192.168.64.1 to 192.168.71.254192.168.71.255
10192.168.72.0192.168.72.1 to 192.168.79.254192.168.79.255
11192.168.80.0192.168.80.1 to 192.168.87.254192.168.87.255
12192.168.88.0192.168.88.1 to 192.168.95.254192.168.95.255
13192.168.96.0192.168.96.1 to 192.168.103.254192.168.103.255
14192.168.104.0192.168.104.1 to 192.168.111.254192.168.111.255
15192.168.112.0192.168.112.1 to 192.168.119.254192.168.119.255
16192.168.120.0192.168.120.1 to 192.168.127.254192.168.127.255
17192.168.128.0192.168.128.1 to 192.168.135.254192.168.135.255
18192.168.136.0192.168.136.1 to 192.168.143.254192.168.143.255
19192.168.144.0192.168.144.1 to 192.168.151.254192.168.151.255
20192.168.152.0192.168.152.1 to 192.168.159.254192.168.159.255
21192.168.160.0192.168.160.1 to 192.168.167.254192.168.167.255
22192.168.168.0192.168.168.1 to 192.168.175.254192.168.175.255
23192.168.176.0192.168.176.1 to 192.168.183.254192.168.183.255
24192.168.184.0192.168.184.1 to 192.168.191.254192.168.191.255
25192.168.192.0192.168.192.1 to 192.168.199.254192.168.199.255
26192.168.200.0192.168.200.1 to 192.168.207.254192.168.207.255
27192.168.208.0192.168.208.1 to 192.168.215.254192.168.215.255
28192.168.216.0192.168.216.1 to 192.168.223.254192.168.223.255
29192.168.224.0192.168.224.1 to 192.168.231.254192.168.231.255
30192.168.232.0192.168.232.1 to 192.168.239.254192.168.239.255
31192.168.240.0192.168.240.1 to 192.168.247.254192.168.247.255
32192.168.248.0192.168.248.1 to 192.168.255.254192.168.255.255

/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.

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/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.

dotted decimal notation for /21 subnet mask

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.

subnetting class B network with /21 subnet mask

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. Therefore, the usable number of subnets will be 32 – 2 = 30.

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.

subnetting class A network with /21 prefix length

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.

Related Contents
/16 subnet maskEverything About /24 Subnet Mask
Everything About /17 Subnet Mask/25 Subnet Mask
Everything About /18 Subnet Mask/26 Subnet Mask
Everything About /19 Subnet Mask/27 Subnet Mask
Everything About /20 Subnet Mask/30 Subnet Mask
/21 Subnet Mask | /21 IP Range/31 Subnet Mask
/22 Subnet Mask/32 Subnet Mask
/23 Subnet Mask/28 Subnet Mask
/29 Subnet Mask

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