Subnetting Operations

Use the following chart to identify the solutions to common subnetting tasks.

Scenario	Solution
Given a network address and subnet mask, how many subnets can you have?	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.
Given a network address and subnet mask, how many hosts per subnet can you have?	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.
Given a network address and customer requirements, what subnet mask should you use?	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.
Given a network address and a subnet mask, identify the valid subnet addresses.	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
Given an IP address and subnet mask, find the: Subnet address Broadcast address Valid host address range	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