“Number Of Valid Subnets” Questions

How many valid subnets exist on the 15.0.0.0 /13 network?

This is a Class A network, with a default mask of /8. The subnet mask is /13, meaning there are five subnet bits. 2 to the 5^th power

(2 x 2 x 2 x 2 x 2) equals 32; subtract 2 from that and 30 valid subnets remain.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask

255.254.0.0 (/13) 11111111 11111000 00000000 00000000

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How many valid subnets exist on the 222.10.1.0 /30 network?

This is a Class C network, with a default mask of /24. The subnet mask is /30, meaning there are six subnet bits. 2 to the 6^th power is 64; subtract 2 from that, and 62 valid subnets remain.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.255.255.0 11111111 11111111 11111111 00000000 Subnet Mask

255.255.255.252 11111111 11111111 11111111 11111100

How many valid subnets exist on the 145.45.0.0 /25 network?

This is a Class B network, with a default mask of /16. The subnet mask is /25, meaning there are nine subnet bits. 2 to the 9^th power is 512; subtract 2 from that, and 510 valid subnets remain.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network

Mask

255.255.0.0

11111111 11111111 00000000 00000000

Subnet Mask

255.255.255.128 11111111 11111111 11111111 10000000

How many valid subnets exist on the 20.0.0.0 255.192.0.0 network?

This is a Class A network, with a default mask of 255.0.0.0. When comparing masks expressed in dotted decimal, it may be helpful to you to convert them to binary to determine the number of subnet bits.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask

255.192.0.0 11111111 11000000 00000000 00000000

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The bits that are set to “0” in the default mask but are set to “1” in the subnet mask are the subnet bits. There are two subnet bits. 2 to the 2^nd power is 4; subtract 2 from that and there are 2 valid subnets left.

How many valid subnets exist on the 130.30.0.0 255.255.224.0 network?

This is a Class B network, with a default mask of 255.255.0.0. When comparing masks expressed in dotted decimal, it may be helpful to convert them to binary to determine the number of subnet bits.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.255.0.0 11111111 11111111 00000000 00000000 Subnet Mask

255.255.224.0 11111111 11111111 11100000 00000000 The bits that are set to “0” in the default mask but are set to “1” in the subnet mask are the subnet bits; there are three of them. 2 to the 3^rd power is 8; subtract 2 from that, and six valid subnets remain.

How many valid subnets exist on the 128.10.0.0 /19 network?

This is a Class B network with a default mask of /16. The subnet mask is /19, indicating there are three subnet bits. 2 to the 3^rd power is 8;

subtract 2 from that, and six valid subnets remain.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.255.0.0 11111111 11111111 00000000 00000000 Subnet Mask

255.255.224.0 11111111 11111111 11100000 00000000

How many valid subnets exist on the 25.0.0.0 /20 network?

This is a Class A network with a default mask of /8. The subnet mask is /20, indicating there are 12 subnet bits. 2 to the 12^th power is 4096; subtract 2 from that, and 4094 valid subnets remain.

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1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask

255.255.240.0 11111111 11111111 11110000 11111100

How many valid subnets exist on the 99.0.0.0 /17 network?

This is a Class A network with a default mask of /8. The subnet mask is /17, indicating there are 9 subnet bits. 2 to the 9^th power is 512;

subtract 2 from that, and 510 valid subnets remain.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask

255.255.128.0 11111111 11111111 10000000 00000000

How many valid subnets exist on the 222.10.8.0 /28 network?

This is a Class C network, with a default mask of /24. The subnet mask is /28, indicating 4 subnet bits. 2 to the 4^th power is 16; subtract 2 from that, and 14 valid subnets remain.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.255.255.0 11111111 11111111 11111111 00000000 Subnet Mask

255.255.255.240 11111111 11111111 11111111 11110000

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How many valid subnets exist on the 20.0.0.0 255.254.0.0 network?

This is a Class A network, with a default mask of 255.0.0.0. To compare the default and subnet masks, convert them to binary:

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask

255.254.0.0 11111111 11111110 00000000 00000000 The bits where the default mask has a “0” and the subnet mask has a

“1” are the subnet bits; there are 7 of them. 2 to the 7^th power is 128; subtract 2 and 126 valid subnets remain.

How many valid subnets exist on the 210.17.90.0 /29 network?

This is a Class C network, with a default mask of /24. The subnet mask is /29, indicating there are 5 subnet bits. 2 to the 5^th power is 32; subtract 2 from that and there are 30 valid subnets.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.255.255.0 11111111 11111111 11111111 00000000 Subnet Mask

255.255.255.248 11111111 11111111 11111111 11111000

How many valid subnets exist on the 130.45.0.0 /26 network?

This is a Class B network, with a default mask of /16. The subnet mask is /26, indicating there are 10 subnet bits. 2 to the 10^th power is 1024; subtract 2 from that and there are 1022 valid subnets.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.255.0.0 11111111 11111111 00000000 00000000 Subnet Mask

255.255.255.192 11111111 11111111 11111111 11000000

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How many valid subnets exist on the 200.1.1.0 /26 network?

This is a Class C network, with a default mask of /24. The subnet mask is /26, indicating there are 2 subnet bits. 2 to the 2^nd power is 4; subtract 2 from that and there are 2 valid subnets.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.255.255.0 11111111 11111111 11111111 00000000 Subnet Mask

255.255.255.192 11111111 11111111 11111111 11000000 How many valid subnets exist on the 45.0.0.0 255.240.0.0 network?

This is a Class A network, with a default mask of 255.0.0.0. To compare the default and subnet masks, convert them to binary.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask

255.240.0.0 11111111 11110000 00000000 00000000 The bits where the default mask has a “0” and the subnet mask has a

“1” are the subnet bits. There are four subnet bits. 2 to the 4^th power is 16; subtract 2 from that and there are 14 valid subnets.

How many valid subnets exist on the 222.33.44.0 255.255.255.248 network?

This is a Class C network, with a default mask of 255.255.255.0. To compare the network and subnet masks, convert them to binary.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.255.255.0 11111111 11111111 11111111 00000000 Subnet Mask

255.255.255.248 11111111 11111111 11111111 11111000 The bits where the default mask has a “0” and the subnet mask has a

“1” are the subnet bits. There are 5 subnet bits. 2 to the 5^th power is 32; subtract 2 and there are 30 valid subnets.

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How many valid subnets exist on the 23.0.0.0 255.255.224.0 network?

This is a Class A network, with a default mask of 255.0.0.0. To compare the network and subnet masks, convert them to binary.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask

255.255.224.0 11111111 11111111 11100000 00000000 The bits where the default mask has a “0” and the subnet mask has a

“1” are the subnet bits. There are 11 subnet bits. 2 to the 11^th power is 2048; subtract 2 from that and there are 2046 valid subnets.

How many valid subnets exist on the 111.11.0.0 /15 network?

This is a Class A network, with a default mask of /8. The subnet mask is /15, indicating there are 7 subnet bits. 2 to the 7^th power is 128;

subtract 2 from that and there are 126 valid subnets.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask

255.254.0.0 11111111 11111110 00000000 00000000

How many valid subnets exist on the 130.45.0.0 /19 network?

This is a Class B network, with a default mask of /16. The subnet mask is /19, indicating there are 3 subnet bits. 2 to the 3^rd power is 8; subtract 2 from that and there are 6 valid subnets.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.255.0.0 11111111 11111111 00000000 00000000 Subnet Mask

255.255.224.0 11111111 11111111 11100000 00000000 Chris Bryant, CCIE #12933

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How many valid subnets exist on the 99.0.0.0 /16 network?

This is a Class A network, with a default mask of /8. The subnet mask is /16, indicating there are 8 subnet bits. 2 to the 8^th power is 256;

subtract 2 from that and there are 254 valid subnets.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask

255.255.0.0 11111111 11111111 00000000 00000000 How many valid subnets exist on the 220.10.78.0 /24 network?

This is a Class C network, with a default mask of /24. The supplied mask is /24 as well; this network has not been subnetted.

How many valid subnets exist on the 34.0.0.0 /18 network?

This is a Class A network, with a default mask of /8. The subnet mask is /18, indicating there are 10 subnet bits. 2 to the 10^th power is

1024; subtract 2 and there are 1022 valid subnets.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask

255.255.192.0 11111111 11111111 11000000 00000000

How many valid subnets exist on the 200.10.56.0 /29 network?

This is a Class C network, with a default mask of /24. The subnet mask is /29, indicating there are 5 subnet bits. 2 to the 5^th power is 32; subtract 2 from that and there are 30 valid subnets.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.255.255.0 11111111 11111111 11111111 00000000 Subnet Mask

255.255.255.248 11111111 11111111 11111111 11111000

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How many valid subnets exist on the 128.10.0.0 /18 network?

This is a Class B network, with a default mask of /16. The subnet mask is /18, indicating there are 2 subnet bits. 2 to the 2^nd power is 4; subtract 2 from that and there are 2 valid subnets.

1^st Octet 2^nd Octet 3^rd Octet 4^th Octet Default Network Mask

255.255.0.0 11111111 11111111 00000000 00000000 Subnet Mask

255.255.192.0 11111111 11111111 11000000 00000000

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Section Five: Determining The Number Of Valid

In document Mastering Binary Math and Subnetting (Page 31-40)