# CCNA Cidr To Subnet Memorization

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ie. . . What is the subnet mask for a CIDR of /25?

This quiz is for the most common CIDR's ranging from /17 - /30

• 1.

### What is the subnet mask for a CIDR of /17? Note: Class B

Explanation
The subnet mask for a CIDR of /17 is 255.255.128.0. In a Class B network, the first two octets are fixed as 255.255, indicating that the network portion is 16 bits. Since the CIDR notation specifies a subnet mask with 17 bits, the third octet must have the first 9 bits set to 1, which gives us 255.255.128.0 as the subnet mask. This allows for 128 subnets and 32,768 host addresses per subnet.

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• 2.

### What is the subnet mask for a CIDR of /18?Note: Class B

Explanation
The subnet mask for a CIDR of /18 is 255.255.192.0. This is because the CIDR notation /18 represents that the first 18 bits of the subnet mask are set to 1, while the remaining bits are set to 0. In this case, the first two octets of the subnet mask (255.255) are fixed, and the third octet (192) has the first 6 bits set to 1, representing a decimal value of 192. The fourth octet (0) has all bits set to 0. Therefore, the subnet mask is 255.255.192.0.

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• 3.

### What is the subnet mask for a CIDR of /19?Note: Class B

Explanation
The subnet mask for a CIDR of /19 is 255.255.224.0. This is because a CIDR of /19 indicates that the first 19 bits of the subnet mask are set to 1, while the remaining bits are set to 0. In this case, the first three octets (24 bits) of the subnet mask are set to 1, representing the network portion, while the last octet (8 bits) is set to 0, representing the host portion. Therefore, the subnet mask is 255.255.224.0.

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• 4.

### What is the subnet mask for a CIDR of /20?Note: Class B

Explanation
The subnet mask for a CIDR of /20 is 255.255.240.0. This subnet mask is used to divide a Class B network into smaller subnetworks. It allows for 4096 subnets and 4094 hosts per subnet. By using this subnet mask, the first 20 bits of the IP address are used to identify the network, while the remaining 12 bits are used to identify the hosts within each subnet.

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• 5.

### What is the subnet mask for a CIDR of /21?Note: Class B

Explanation
The subnet mask for a CIDR of /21 is 255.255.248.0. This subnet mask is used for Class B networks and allows for a total of 2,048 IP addresses. The CIDR notation /21 indicates that the first 21 bits of the IP address are used for the network portion, leaving 11 bits for the host portion. The subnet mask 255.255.248.0 has the first 21 bits set to 1, indicating the network portion, and the remaining 11 bits set to 0, indicating the host portion.

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• 6.

### What is the subnet mask for a CIDR of /22?Note: Class B

Explanation
The subnet mask for a CIDR of /22 in a Class B network is 255.255.252.0. This subnet mask allows for 1024 IP addresses, with 22 bits used for the network portion and 10 bits used for the host portion. The first 22 bits are set to 1 in the subnet mask, indicating the network address, and the remaining 10 bits are set to 0, indicating the host address.

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• 7.

### What is the subnet mask for a CIDR of /23?Note: Class B

Explanation
The subnet mask for a CIDR of /23 in a Class B network is 255.255.254.0. This subnet mask allows for 9 bits of network address and 7 bits of host address, resulting in a total of 512 subnets and 512 hosts per subnet.

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• 8.

### What is the subnet mask for a CIDR of /24?Note: Class C

Explanation
The subnet mask for a CIDR of /24 in a Class C network is 255.255.255.0. This subnet mask is used to divide the IP address into network and host portions. In a Class C network, the first three octets (255.255.255) are fixed, indicating the network portion, while the last octet (0) represents the host portion. This subnet mask allows for a maximum of 254 hosts to be connected to the network.

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• 9.

### What is the subnet mask for a CIDR of /25? Note: Class C

Explanation
The subnet mask for a CIDR of /25 is 255.255.255.128. In CIDR notation, the number after the slash represents the number of bits that are used for the network portion of the IP address. In this case, /25 means that the first 25 bits are used for the network portion, leaving 7 bits for the host portion. The subnet mask is a binary value that has 1s for the network portion and 0s for the host portion. When converted to decimal, it becomes 255.255.255.128.

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• 10.

### What is the subnet mask for a CIDR of /26? Note: Class C

Explanation
The subnet mask for a CIDR of /26 is 255.255.255.192. This subnet mask is used for Class C networks, which have a default subnet mask of 255.255.255.0. By using a CIDR of /26, we are borrowing 6 bits from the host portion of the IP address to create subnets. This results in a subnet mask of 255.255.255.192, where the first 26 bits are set to 1 and the remaining 6 bits are set to 0. This allows for a total of 64 subnets and 62 hosts per subnet.

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• 11.

### What is the subnet mask for a CIDR of /27? Note: Class C

Explanation
The subnet mask for a CIDR of /27 is 255.255.255.224. This subnet mask is used to divide a network into smaller subnets. In a /27 CIDR notation, the first 27 bits of the subnet mask are set to 1, indicating the network portion of the IP address, while the remaining 5 bits are set to 0, indicating the host portion. This allows for a total of 32 subnets and 30 hosts per subnet in a Class C network.

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• 12.

### What is the subnet mask for a CIDR of /28? Note: Class C

Explanation
The subnet mask for a CIDR of /28 is 255.255.255.240. This subnet mask is used in Class C networks and allows for 16 subnets and 14 hosts per subnet. The CIDR notation /28 means that the first 28 bits of the subnet mask are set to 1, indicating the network portion, while the remaining 4 bits are set to 0, indicating the host portion. This subnet mask provides a fine-grained level of network segmentation and efficient utilization of IP addresses.

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• 13.

### What is the subnet mask for a CIDR of /29? Note: Class C

Explanation
The subnet mask for a CIDR of /29 is 255.255.255.248. This subnet mask is used for class C networks and allows for 8 IP addresses, with 3 bits borrowed from the host portion of the IP address to create the subnet.

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• 14.

### What is the subnet mask for a CIDR of /30? Note: Class C

Explanation
The subnet mask for a CIDR of /30 is 255.255.255.252. This is because a CIDR of /30 allows for 4 IP addresses, with 2 usable addresses for hosts. The subnet mask 255.255.255.252 provides a network size of 4 IP addresses, with 2 addresses for hosts and 2 addresses for network and broadcast.

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• Current Version
• Mar 22, 2023
Quiz Edited by
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• Nov 15, 2009
Quiz Created by
Rodney.butler

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