How many valid host addresses exist in the 220.11.10.0 /26 subnet?
This is a Class C network, with a default mask of /24. The subnet mask is /26, indicating that there are 2 subnet bits. With 24 network bits and 2 subnet bits, that leaves 6 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.255.255.0 11111111 11111111 11111111 00000000 Subnet Mask
255.255.255.192 (/26)
11111111 11111111 11111111 11000000
2 to the 6th power equals 64; subtract 2 from that, and 62 valid host addresses remain.
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How many valid host addresses exist in the 129.15.0.0 /21 subnet?
This is a Class B network, with a default mask of /16. The subnet mask is /21, indicating there are 5 subnet bits. With 16 network bits and 5 subnet bits, that leaves 11 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.255.0.0 11111111 11111111 00000000 00000000 Subnet Mask
255.255.248.0 (/21) 11111111 11111111 11111000 00000000 2 to the 11th power equals 2048; subtract 2 from that and 2046 valid host addresses remain.
How many valid host addresses exist in the 50.0.0.0 /20 subnet?
This is a Class A network, with a default mask of /8. The subnet mask is /20, indicating there are 12 subnet bits. This leaves 12 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.0.0.0 11111111 11111111 00000000 00000000 Subnet Mask
255.255.240.0 (/20) 11111111 11111111 11110000 00000000 2 to the 12th power equals 4096; subtract 2 from that and 4094 valid host addresses remain.
How many valid host addresses exist in the 222.22.2.0 /30 subnet?
This is a Class C network, with a default mask of /24. The subnet mask is /30, indicating there are 6 subnet bits. This leaves 2 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.255.255.0 11111111 11111111 11111111 00000000 Subnet Mask
255.255.255.252 11111111 11111111 11111111 11111100 Chris Bryant, CCIE #12933
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2 to the 2nd power equals 4; subtract 2 from that and 2 valid host addresses remain. (This is a common subnet mask for point-to-point connections.)
How many valid host addresses exist in the 212.10.3.0 /28 subnet?
This is a Class C network, with a default mask of /24. The subnet mask is /28, indicating there are 4 subnet bits. This leaves 4 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.255.255.0 11111111 11111111 11111111 00000000 Subnet Mask
255.255.255.240 (/28) 11111111 11111111 11111111 11110000 2 to the 4th power equals 16; subtract 2 from that and 14 valid host addresses remain.
How many valid addresses exist in the 14.0.0.0 /20 subnet?
This is a Class A network, with a default mask of /8. The subnet mask is /20, indicating there are 12 subnet bits. This leaves 12 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask
255.255.240.0 (/20) 11111111 11111111 11110000 00000000 2 to the 12th power is 4096; subtract 2 from that and 4094 valid host addresses remain.
How many valid host addresses exist in the 45.0.0.0 255.255.192.0 subnet?
This is a Class A network, with a default mask of /8, or 255.0.0.0.
Comparing the subnet mask to the default mask in binary form reveals that there are 10 subnet bits and 14 host bits:
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1st Octet 2nd Octet 3rd Octet 4th Octet Default Network
Mask 255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask
255.255.192.0 (/18) 11111111 11111111 11000000 00000000 2 to the 14th power is 16384; subtract 2 from that, and 16382 valid host addresses remain.
How many valid host addresses exist on the 221.10.78.0 255.255.255.224 subnet?
This is a Class C network, with a default mask of /24, or
255.255.255.0. Comparing the subnet mask to the default mask in binary form reveals that there are 3 subnet bits and 5 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.255.255.0 11111111 11111111 11111111 00000000 Subnet Mask
255.255.255.224 11111111 11111111 11111111 11100000 2 to the 5th power is 32; subtract 2 from that, and 30 valid host
addresses remain.
How many valid host addresses exist on the 143.34.0.0 255.255.255.192 subnet?
This is a Class B network, with a default mask of /16, or 255.255.0.0.
Comparing the subnet mask to the default mask in binary form reveals that there are 10 subnet bits and 6 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.255.0.0 11111111 11111111 00000000 00000000 Subnet Mask
255.255.255.192 11111111 11111111 11111111 11000000 2 to the 6th power is 64; subtract 2 from that, and 62 valid host
addresses remain.
Chris Bryant, CCIE #12933 www.thebryantadvantage.com
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© 2005 The Bryant Advantage
How many valid host addresses exist on the 128.12.0.0 255.255.255.240 subnet?
This is a Class B network, with a default mask of /16, or 255.255.0.0.
Comparing the subnet mask to the default mask in binary form reveals that there are 4 subnet bits and 4 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.255.0.0 11111111 11111111 00000000 00000000 Subnet Mask
255.255.255.240 11111111 11111111 11111111 11110000 2 to the 4th power is 16; subtract 2 from that, and 14 valid host
addresses remain.
How many valid host addresses exist on the 125.0.0.0 /24 subnet?
This is a Class A network, with a default mask of /8, or 255.0.0.0.
Comparing the default and subnet masks in binary reveals that there are 16 subnet bits and 8 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.0.0.0 11111111 00000000 00000000 00000000 Subnet Mask
255.255.255.0 11111111 11111111 11111111 00000000 2 to the 8th power is 256; subtract 2 from that, and 254 valid host addresses remain.
Chris Bryant, CCIE #12933 www.thebryantadvantage.com
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How many valid addresses exist on the 221.10.89.0 255.255.255.248 subnet?
This is a Class C network, with a default mask of 255.255.255.0, or /24. Comparing the default and subnet masks in binary reveals that there are 5 subnet bits and 3 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.255.255.0 11111111 11111111 11111111 00000000 Subnet Mask
255.255.255.248 11111111 11111111 11111111 11111000 2 to the 3rd power is 8; subtract 2 from that, and 6 valid host
addresses remain.
How many valid addresses exist on the 134.45.0.0 /22 subnet?
This is a Class B network, with a default mask of /16, or 255.255.0.0.
The subnet mask is /22, indicating that 6 subnet bits have been set.
With 16 network bits and 6 subnet bits set, that leaves 10 host bits:
1st Octet 2nd Octet 3rd Octet 4th Octet Default Network Mask
255.255.0.0 11111111 11111111 00000000 00000000 Subnet Mask
255.255.252.0 (/22) 11111111 11111111 11111100 00000000 2 to the 10th power is 1024; subtract 2 from that and 1022 valid host addresses remain.
As you’ve now seen, determining the number of valid subnets or valid hosts is a matter of breaking the masks down into binary and
determining how many subnet bits and host bits there are.
As your skill increases, you’ll be able to answer some of these questions without breaking the masks down into binary. I strongly recommend that you do break them down into binary whenever possible, though. As you progress through the Cisco certifications, you’ll see subnet masks that are progressively more complex.
Developing the habit of breaking masks down into binary will be key in your success not only on the CCNA exam, but CCNP and CCIE exams as well.
Chris Bryant, CCIE #12933 www.thebryantadvantage.com
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