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Techniques for Solving Subnetting Questions

Techniques for Solving Subnetting Questions. Common questions types looked at Complicated questions section added. Basic Types of Questions. First let us define the Basics. Basic Types of Questions. These are the basics you should be comfortable with

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Techniques for Solving Subnetting Questions

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  1. Techniques for Solving Subnetting Questions Common questions types looked at Complicated questions section added ADR@2007

  2. Basic Types of Questions First let us define the Basics ADR@2007

  3. 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 == 255.0.0.0 • 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 == 255.255.255.224 • ANDing e.g. 192.168.100.5 AND 255.255.255.224 = 192.168.100.0 ADR@2007

  4. More complicated question types Now let’s explore methods for dealing with trickier subnetting questions ADR@2007

  5. Methods for Tackling Subnetting Questions • Formula (subnet or host) Method • Calculate a Subnet Mask • Given number of subnets needed or • Given number of hosts needed • Calculate Number of Addresses per Subnet • Given an address and subnet mask • AND Method • Calculate a Subnet Address • Given an IP address and mask • AND/Formula Method • Calculate Type of Address • Given an IP address and mask • Pie Chart Method • Use for any of the above types of questions (class C networks) ADR@2007

  6. Subnet Formula Method Calculate a Subnet Mask • Asked to • calculate a subnet mask • Given • number of subnets needed ADR@2007

  7. Subnet Formula Method • Asked to • Calculate a Subnet Mask • Given • The Number of Subnets Needed ANS 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? 255.255.0.0 255.255.240.0 255.255.255.128 255.255.254.0 255.255.255.192 ADR@2007

  8. Subnet Formula Method: Steps To calculate the subnet mask you need figure how many subnet bits must be borrowed. Use the formula…2n = 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 ADR@2007

  9. Host Formula Method Calculate a Subnet Mask • Asked to • calculate a subnet mask • Given • number of hosts per subnet ADR@2007

  10. Host FormulaMethod • Asked to • Calculate a Subnet Mask • Given • Number of Hosts per Subnet ANS 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? 255.255.255.252 255.255.255.248 255.255.255.240 255.255.255.224 255.255.255.128 ADR@2007

  11. Host Formula Method: Steps • To calculate the subnet mask, you need to figure out how many host bits are left. • Use the formula…2h -2 = number of hosts to calculate h, the number of host bits • Calculate the CIDR mask • Convert the CIDR mask to full subnet mask 2h = total number addresses 2h - 2 = useable number addresses ADR@2007

  12. Host Formula Method Reversed Calculate Number of Addresses per Subnet • Asked to • calculate the number of addresses in each subnet (total or useable) • Given • address and mask ADR@2007

  13. Host Formula Method Reversed • Asked to • Calculate Number Addresses per Subnet • Given • Address and Mask ANS Company XYZ uses a network address of 192.168.50.0. It uses the mask of 255.255.255.192 to create subnets. What is the maximum number of usable hosts in each subnet? 6 14 30 62 ADR@2007

  14. Host Formula Method Reversed: Steps • Examine the subnet mask and determine how many host bits (h) there are. • Use 2h formula to calculate total number addresses per subnet or • Use 2h - 2 formula to calculate useable number of addresses per subnet i.e. number of hosts = 2h -2 ADR@2007

  15. ANDING Method Calculate Subnet Addresses • Asked to • calculate a subnet address • Given • an IP address and mask ADR@2007

  16. ANDING Method • Asked to • Calculate a Subnet Address • Given • An IP Address and Mask ANS Which address is a valid subnet address if a 26 bit mask is used for subnetting? 172.16.192.252 172.16.128.32 172.16.43.16 172.16.243.64 172.16.157.96 ADR@2007

  17. 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 ADR@2007

  18. ANDING/FormulaMethodCalculate Address Type • Asked to • Calculate the Type of Address • Given • An IP Address and Mask ADR@2007

  19. ANDING/Formula Method • Asked to • Calculate the Type of Address • Given • An IP Address and Mask ANS Which type of address is 192.168.110.112/28? host address subnet address broadcast address multicast address ADR@2007

  20. ANDING/Formula Method: Steps AND the IP address and subnet mask to find the home subnetDoes the answer give you the same address? If yes then the IP address is a subnet address. Use the formula (2h ) 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. ADR@2007

  21. 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 2n = 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 2h -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 = 2h -2 ADR@2007

  22. 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 • 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 2h 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 ADR@2007

  23. Determine the method to usefor the following questions ADR@2007

  24. What Question Type is This? • Host Formula Reversed Method • Asked to • Calculate Number Addresses per Subnet • Given • Network Address and Mask Click to show method ANS When a network administrator applies the subnet mask 255.255.255.252 to a Class A address, for any given subnet, how many IP addresses are available to be assigned to devices? ADR@2007

  25. What Question Type is This? • ANDING Method • Asked to • Calculate the Subnet an Address Belongs To • Given • An IP Address and Mask Click to show method ANS A router interface has been assigned an IP address of 172.16.30.166 with a mask of 255.255.255.248. To which subnet does the IP address belong? ADR@2007

  26. What Question Type is This? • Host Formula Method • Asked to • Calculate a Subnet Mask • Given • Number of Hosts per Subnet Click to show method ANS 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? 255.255.255.248 255.255.255.192 255.255.255.240 ADR@2007

  27. What Question Type is This? • ANDING/Formula Method • Asked to • Calculate the Type of Address • Given • An IP Address and Mask Click to show method ANS Which type of address is 192.168.110.127/28? host address subnet address broadcast address multicast address ADR@2007

  28. What Question Type is This? • Formula Method (host and subnet) • Asked to • Calculate a Subnet Mask • Given • Number of Hosts per Subnet & Number Subnets Click to show method ANS 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? 255.255.255.192 255.255.255.248 255.255.255.224 255.255.224.0 ADR@2007

  29. 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. ADR@2007

  30. Complicated Example 1 ANS The addressing scheme for the LAN is 172.25.24.0/26. The hosts will be in the first subnet. The LAN interface of the router is to be configured using 172.25.24.63 as the IP address. The LAN is being addressed properly. The router LAN interface is being assigned a subnet address. The router LAN interface is being assigned a broadcast address. The subnet mask does not allow enough host addresses in a single subnet ADR@2007

  31. 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 ADR@2007

  32. Complicated Example 2 ANS The addressing scheme shown above is being planned for an office. What is the status of the proposed network? The configuration will work as planned. The subnet mask of host A is incorrect. The default gateway of host A is a network address. The addresses on the router LAN interfaces are on the same subnet. The IP address of host A is on a different subnet than the subnet that the Ethernet router interface ADR@2007

  33. 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 ADR@2007

  34. Complicated Example 3 ANS Host A is connected to the LAN, but it cannot connect to the Internet. What are the two problems with this configuration? The default gateway is a network address. The default gateway is on a different network than the host. The router serial interface is on the same network as the LAN interface The host subnet mask is incorrect. ADR@2007

  35. 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 ADR@2007

  36. Complicated Example 3 7 0 Work out how many subnets for /27 3 bits = 8 subnetsdraw subnets on diagram 6 1 Work out how many addresses in each subnet A 5 2 5 bits left = 32 addresses in eachLabel subnet addresses diagram .192 .224 .160 .64 .128 .96 .32 4 3 Which subnet is host .65 on? Host .65 is on subnet 2 Work out the full subnet mask for /27 Which subnet is .33 gateway on? Gateway .33 is on subnet 1 Subnet mask is 255.255.255.224 Draw a diagram click for each step ADR@2007

  37. Complicated Example 4 ANS For network 192.168.10.0/28, 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? IP address: 192.168.10.78 mask: 255.255.255.240 gateway: 192.168.10.79 IP address: 192.168.10.78 mask: 255.255.255.240 gateway: 192.168.10.65 IP address: 192.168.10.79 mask: 255.255.255.248 gateway: 192.168.10.62 IP address: 192.168.10.254 mask: 255.255.255.0 gateway: 192.168.10.1 ADR@2007

  38. 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 ADR@2007

  39. Complicated Example 4 0 Work out how many subnets for /28 1 4 bits = 16 subnets draw subnets on diagram 2 3 Work out the full subnet mask for /28 4 Subnet mask is 255.255.255.240 5 .79 Br addr .64 Sub addr .80 Sub addr 6 7 Work out how many addresses in each subnet 4 bits left = 16 addresses in each Label fifth subnet on diagram Assign last address in fifth subnet to server Assign first address in fifth subnet to gateway Last address is .78 First address is .65 Draw a diagram click for each step ADR@2007

  40. End of Presentation practice, practice, practice ADR@2007

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