Techniques for Solving Subnetting Questions Common questions types looked at Complicated questions section added

Basic Types of Questions 2 First let us define the Basics

Basic Types of Questions These are the basics you should be comfortable with Knowing the default masks for the 3 network classes e.g. Class A == /8 == The default number of network and host bits for each class e.g. Class A == 8 network bits == 24 host bits Converting a CIDR mask to a subnet mask and vice-versa. e.g. /27 == ANDing e.g AND =

More complicated question types 4 Now let’s explore methods for dealing with trickier subnetting questions

Methods for Tackling Subnetting Questions Formula (subnet or host) Method Calculate a Subnet Mask o Given number of subnets needed or o Given number of hosts needed Calculate Number of Addresses per Subnet o Given an address and subnet mask AND Method Calculate a Subnet Address o Given an IP address and mask o AND/Formula Method Calculate Type of Address o Given an IP address and mask Pie Chart Method Use for any of the above types of questions (class C networks) 5

Subnet Formula Method Calculate a Subnet Mask Asked to o calculate a subnet mask Given o number of subnets needed 6

Subnet Formula Method A company is using a Class B IP addressing scheme and expects to need as many as 80 networks. What is the correct subnet mask to use with the network configuration? a b c d e ANS Asked to o Calculate a Subnet Mask Given o The Number of Subnets Needed Asked to o Calculate a Subnet Mask Given o The Number of Subnets Needed 7

To calculate the subnet mask you need figure how many subnet bits must be borrowed. Use the formula… 2 n = number of subnets to calculate n, which is the number of bits you need to borrow to obtain the required number of subnets. Calculate the CIDR mask Convert the CIDR mask to full subnet mask 8 Subnet Formula Method: Steps

Host Formula Method Calculate a Subnet Mask Asked to o calculate a subnet mask Given o number of hosts per subnet 9

An IP network address has been subnetted so that every subnetwork has 30 usable host IP addresses. What is the appropriate subnet mask for the newly created subnetworks? a b c d e Asked to o Calculate a Subnet Mask Given o Number of Hosts per Subnet Asked to o Calculate a Subnet Mask Given o Number of Hosts per Subnet ANS 10 Host Formula Method

To calculate the subnet mask, you need to figure out how many host bits are left. Use the formula… 2 h -2 = number of hosts to calculate h, the number of host bits Calculate the CIDR mask Convert the CIDR mask to full subnet mask 2 h = total number addresses 2 h - 2 = useable number addresses 11 Host Formula Method: Steps

Host Formula Method Reversed Calculate Number of Addresses per Subnet Asked to o calculate the number of addresses in each subnet (total or useable) Given o address and mask 12

Company XYZ uses a network address of It uses the mask of to create subnets. What is the maximum number of usable hosts in each subnet? a.6 b.14 c.30 d.62 Asked to o Calculate Number Addresses per Subnet Given o Address and Mask Asked to o Calculate Number Addresses per Subnet Given o Address and Mask ANS 13 Host Formula Method Reversed

Examine the subnet mask and determine how many host bits (h) there are. Use 2 h formula to calculate total number addresses per subnet or Use 2 h - 2 formula to calculate useable number of addresses per subnet i.e. number of hosts = 2 h Host Formula Method Reversed: Steps

Asked to o calculate a subnet address Given o an IP address and mask 15 ANDING Method Calculate Subnet Addresses

Which address is a valid subnet address if a 26 bit mask is used for subnetting? a b c d e Asked to o Calculate a Subnet Address Given o An IP Address and Mask Asked to o Calculate a Subnet Address Given o An IP Address and Mask ANS 16 ANDING Method

Convert each IP address to binary Convert the subnet mask to binary AND each address against the mask You will know if an IP address is actually a subnet address because the answer will be the same address. Hint: all subnet addresses are even numbers. All broadcast addresses are odd numbers 17 ANDING Method

Asked to o Calculate the Type of Address Given o An IP Address and Mask 18 ANDING/Formula Method Calculate Address Type

Which type of address is /28? a.host address b.subnet address c.broadcast address d.multicast address Asked to o Calculate the Type of Address Given o An IP Address and Mask Asked to o Calculate the Type of Address Given o An IP Address and Mask ANS 19 ANDING/Formula Method

AND the IP address and subnet mask to find the home subnet Does the answer give you the same address? If yes then the IP address is a subnet address. Use the formula (2 h ) to work out how many addresses are in each subnet. You need to do this so you can calculate the start of the next subnet. Is the IP address the last address of the subnet. If yes then the address is a broadcast address If the IP address is none of the above then it is an ordinary IP address. 20 ANDING/Formula Method: Steps

Methods Summary Formula (host or subnet ) Method To calculate a subnet mask, when you are given the number of subnets needed use the formula below, then work out the subnet mask 2 n = number subnets To calculate a subnet mask when you are given the number of hosts needed Use the formula below then work out the subnet mask 2 h -2 = number hosts To calculate the number of hosts per subnet, when you are given an address and subnet mask Use the subnet mask to work out how many bits are host bits, then use the formula below number hosts = 2 h -2 21

Methods Summary AND Method To calculate the resident subnet when given an IP address and mask AND the IP address and mask to find the resident subnet o AND/Formula Method Type To work out if an address is a subnet, broadcast or ordinary IP address AND the IP address and mask to find the resident subnet Use the 2 h formula to find the size of the subnet. Add this number -1 to the resident subnet to find the broadcast address Use the above information to decide on the type of IP address Pie Chart Method Use this method for any of the above types of questions for class C networks or when a question is particularly complicated 22

Determine the method to use for the following questions

What Question Type is This? When a network administrator applies the subnet mask to a Class A address, for any given subnet, how many IP addresses are available to be assigned to devices? Host Formula Reversed Method Asked to o Calculate Number Addresses per Subnet Given o Network Address and Mask Host Formula Reversed Method Asked to o Calculate Number Addresses per Subnet Given o Network Address and Mask ANS Click to show method a.510 b.254 c.30 d.14 e.6 f.2 24

What Question Type is This? A router interface has been assigned an IP address of with a mask of To which subnet does the IP address belong? ANDING Method Asked to o Calculate the Subnet an Address Belongs To Given o An IP Address and Mask ANDING Method Asked to o Calculate the Subnet an Address Belongs To Given o An IP Address and Mask ANS a b c d e f Click to show method 25

What Question Type is This? A company is planning to subnet its network for a maximum of 27 hosts. Which subnet mask would provide the needed hosts and leave the fewest unused addresses in each subnet? Host Formula Method Asked to o Calculate a Subnet Mask Given o Number of Hosts per Subnet Host Formula Method Asked to o Calculate a Subnet Mask Given o Number of Hosts per Subnet ANS Click to show method 26

What Question Type is This? Which type of address is /28? host address subnet address broadcast address multicast address ANDING/Formula Method Asked to o Calculate the Type of Address Given o An IP Address and Mask ANDING/Formula Method Asked to o Calculate the Type of Address Given o An IP Address and Mask ANS Click to show method 27

What Question Type is This? A small company has a class C network license and needs to create five usable subnets, each subnet capable of accommodating at least 18 hosts. Which of the following is the appropriate subnet mask? Formula Method (host and subnet) Asked to o Calculate a Subnet Mask Given o Number of Hosts per Subnet & Number Subnets Formula Method (host and subnet) Asked to o Calculate a Subnet Mask Given o Number of Hosts per Subnet & Number Subnets ANS Click to show method 28

Complicated Types Questions These questions types usually involve a diagram and may require combinations of the methods looked at previously. The ANDING method is most often needed, although using a pie chart can make it easier. 29

Complicated Example 1 The addressing scheme for the LAN is /26. The hosts will be in the first subnet. The LAN interface of the router is to be configured using as the IP address. a.The LAN is being addressed properly. b.The router LAN interface is being assigned a subnet address. c.The router LAN interface is being assigned a broadcast address. d.The subnet mask does not allow enough host addresses in a single subnet ANS 30

Complicated Example 1 Need to check hosts and gateway are in same subnet The diagram specifies the network address and mask Hosts addresses are given so you can work out which subnet they are in. Need to check they are all in the same subnet and subnet has enough room for all of them. Gateway address is given so you must check host and gateway are in same subnet. Use ANDING Method to obtain the answer or use a pie chart 31

Complicated Example 2 The addressing scheme shown above is being planned for an office. What is the status of the proposed network? a.The configuration will work as planned. b.The subnet mask of host A is incorrect. c.The default gateway of host A is a network address. d.The addresses on the router LAN interfaces are on the same subnet. e.The IP address of host A is on a different subnet than the subnet that the Ethernet router interface ANS 32

Complicated Example 2 Need to check host, server and router interfaces are in appropriate subnets. Also must check host config How many subnets are there? Are the host and the gateway in the same subnet Is the host config correct. – Is a valid IP address being used – Is a valid mask being used – Is a valid gateway address being used and does it match the address on the router Is the gateway address a valid address Is the subnet address between the router and the server valid? Is it on a separate subnet to the subnet with the host. Is the server address in the correct subnet and is it a valid address Use ANDING Method to obtain the answer or use a pie chart 33

Complicated Example 3 Host A is connected to the LAN, but it cannot connect to the Internet. What are the two problems with this configuration? a.The default gateway is a network address. b.The default gateway is on a different network than the host. c.The router serial interface is on the same network as the LAN interface d.The host subnet mask is incorrect. ANS 34

Complicated Example 3 Check host and router interfaces are in appropriate subnets. Also check host config How many subnets are there? Are the host and the gateway in the same subnet Is the host config correct. – Is a valid IP address being used – Is a valid mask being used – Is a valid gateway address being used and does it match the address on the router Is the gateway address a valid address Is the serial address on a different subnet or network. Is the server address in the correct subnet and is it a valid address Use ANDING Method to obtain the answer or use a pie chart 35

Complicated Example 3 Draw a diagram click for each step.64 1.Work out how many subnets for /27 3 bits = 8 subnets draw subnets on diagram 2.Work out how many addresses in each subnet 5 bits left = 32 addresses in each Label subnet addresses diagram 3.Which subnet is host.65 on? Host.65 is on subnet 2 4.Which subnet is.33 gateway on? Gateway.33 is on subnet 1 5.Work out the full subnet mask for /27 Subnet mask is A

Complicated Example 4 For network /28, an Administrator has decided he wants to use the fifth subnet (subnet zero is the first subnet). The router interface will be assigned the first usable address on the subnet and the server will be given the last usable address. How should the server be configured? a.IP address: mask: gateway: b.IP address: mask: gateway: c.IP address: mask: gateway: d.IP address: mask: gateway: ANS 37

Complicated Example 4 Need to work out the subnet addresses How many subnets are there? How many addresses are in each subnet Work out the address of the fifth subnet Work out the address of the sixth subnet Work out the host config – IP address – mask – gateway address Easiest approach here is to use a pie chart 38

Complicated Example 4 Draw a diagram click for each step.64 Sub addr.80 Sub addr.79 Br addr 1.Work out how many subnets for /28 4 bits = 16 subnets draw subnets on diagram 3.Work out how many addresses in each subnet 4 bits left = 16 addresses in each Label fifth subnet on diagram 4.Assign first address in fifth subnet to gateway First address is.65 5.Assign last address in fifth subnet to server Last address is.78 2.Work out the full subnet mask for /28 Subnet mask is

End of Presentation practice, practice, practice