You have a class C address range and are planning a network that has an average of 50 hosts per subnet.
How many host bits will have to be borrowed for subnetting so that the maximum number of subnets can be
implemented?
A.
1
B.
2
C.
3
D.
4
E.
6
Explanation:
A class C address has 8 bits in host space. By using 2 bits from the host space for subnetting, leaving 6 host
bits, you can create subnets that can accommodate up to 62 hosts each (26 – 2 = 62). This will ensure that the
requirement of 50 hosts per subnet is met and the maximum number of subnets is provided.
The formulas to calculate the number of subnets and hosts are:
Number of subnets = 2number-of-subnet-bits
Number of hosts per subnet = 2number-of-host-bits – 2
If you take 1 bit for subnetting:
Number of subnets = 21 = 2
Number of hosts per subnet = 27 – 2 = 126
This results in a mask of 255.255.255.128 or /25. Since each subnet need not be bigger than 50, this solution
would not maximize the number of subnets.
If you take 2 bits for subnetting:
Number of subnets = 22 = 4
Number of hosts per subnet = 26 – 2 = 62
This results in a mask of 255.255.255.192 or /26. This solution would create more subnets, but the subnets are
smaller than the requirement.
If you take 3 bits for subnetting:
Number of subnets = 23 = 8
Number of hosts per subnet = 25 – 2 = 30
This results in a mask of 255.255.255.224 or /27. This would create more subnets, but the subnets are smaller
than the requirement.
If you take 4 bits for subnetting:
Number of subnets = 24 = 16
Number of hosts per subnet = 24 – 2 = 14
This results in a mask of 255.255.255.240 or /28. This solution would create more subnets, but the subnets are
smaller than the requirement.
If you take 6 bits for subnetting:
Number of subnets = 26 = 64
Number of hosts per subnet = 22 – 2 = 2
This mask, 255.255.255.252 or /30, yields only 2 IP addresses, but is quite commonly used on a point-to-point
link, such as between two routers. This solution would create more subnets, but the subnets are smaller than
the requirement.
You will always subtract 2 from the number of hosts (the formula of 2number-of-host-bits – 2) because the allzeros bit address is reserved for the network address and the all-ones bit address is reserved for the broadcast
address.Prior to Cisco IOS Software Release 12.0, it was common practice to subtract 2 from the networks formula
(2number-of-subnet-bits) to exclude addresses of all 1s and all 0s (called the all-ones subnet and subnet zero).
Today that range is usable, except with some legacy systems. On certain networks with legacy software, you
may need to use the previous formula (2n – 2) to calculate the number of subnets.
Objective:
Network Fundamentals
Sub-Objective:
Apply troubleshooting methodologies to resolve problemsCisco > Technology Support > IP > IP Routing > Design TechNotes > IP Addressing and Subnetting for New
Users > Document ID: 13788 > Understanding IP Addresses
Cisco > Technology Support > IP > IP Addressing Services > Design TechNotes > Subnet Zero and the AllOnes Subnet > Document ID: 13711