Subnetting Operations
Use the following chart to identify the solutions to common subnetting tasks.
Scenario
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Solution
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Given a network address and subnet mask, how many subnets can you have?
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Begin by converting the subnet mask to a binary number. Then decide which formula to use (n is the number of additional bits borrowed from the default mask):
Use 2n if:
- The network uses a classless routing protocol, such as RIP version 2, EIGRP, or OSPF
- The ip subnet zero command is configured
- Variable-length Subnet Mask (VLSM) is used
Use 2n-2 if:
- The network uses a classful routing protocol, such as RIP version 1 or IGRP
- The no ip subnet zero command is configured
Note: If no network details are provided, use 2n.
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Given a network address and subnet mask, how many hosts per subnet can you have?
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2n-2 Begin by converting the subnet mask to a binary number. Then use the formula to find the number of hosts. To find the number of valid hosts, n = the number of unmasked bits by the custom mask.
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Given a network address and customer requirements, what subnet mask should you use?
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2n, 2n-2 Write out the default subnet mask in binary. Then borrow bits and use the formula to find the number that gives you enough subnets and hosts.
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Given a network address and a subnet mask, identify the valid subnet addresses.
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Magic number The magic number is the decimal value of the last 1 bit in the subnet mask. The magic number identifies:
- The first valid subnet address
- The increment value to find additional subnet addresses
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Given an IP address and subnet mask, find the:
- Subnet address
- Broadcast address
- Valid host address range
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Trust the line Use the following process to find the information you need:
- Identify the subnet and host portions of the mask, draw a line
- To find the subnet address, set all host bits to 0
- To find the broadcast address, set all host bits to 1
- The valid host range is:
- First address = Subnet address + 1
- Last address = Broadcast address - 1
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Credit: Testout 640-802 CCNA Notes