Lab 10.7.7 Subnet Mask 

Estimated time: 45 min.

Objectives:

This lab will focus on your ability to accomplish the following tasks:

  • Work with a more complex Class C subnet scenario
  • Determine the subnets available with a particular IP network address and subnet mask
  • Given a network address and requirements, be able to determine how many subnets and hosts
  • Be able to determine what subnet mask should be used to give the appropriate number of hosts and subnets
  • Assign IP addresses and subnet masks to hosts and router interfaces
  • Use the "ANDing" process to move an IP packet from a local host to a remote host through a router

Background:

This lab will build on Lab Subnet Mask 1 and help develop a better understanding of IP subnet masks using a real-world example with additional worksheet exercises based on foundations established in the prior lab.  This lab will focus on a Class C network with three subnets and using a Custom Subnet Mask .

Tools / Preparation:

This is primarily a written lab exercise but you will want to use Control Panel / Network to review some real network IP addresses and the basics covered in the prior lab. The following resources will be required:

  • PC workstation with Windows operating system (Win 95, 98, NT or 2000) installed on the PC and access to the Windows Calculator.

Notes:

Step 1 – IP Address Basics

Explanation: For reference, the IP addressing table from the prior lab is included here. IP network addresses are assigned by the Internet Network Information Center (InterNIC). You will work with a Class C.

Cls 1st Octet Decimal Range 1st Octet High Order Bits Network / Host ID (N=Network, H=Host) Default Subnet Mask Number of Networks Hosts per Network (usable addresses)
A 1 – 126* 0 N.H.H.H 255.0.0.0 126 (27 – 2) 16,777,214 (2 24 – 2)
B 128 – 191 1 0  N.N.H.H 255.255.0.0 16,382 (214 - 2) 65,534 (2 16 – 2)
C 192 – 223 1 1 0 N.N.N.H 255.255.255.0 2,097,150 (221 – 2) 254 (2 8 – 2)
D 224 – 239 1 1 1 0 Reserved for Multicasting
E 240 – 254  1 1 1 1 0 Experimental, used for research

 

Step 2 – Class C network address with 3 subnets.

Task: Use the following information and use the information from the worksheet in the prior lab to help determine your valid subnets and host IP addresses. Do NOT use the zero or last subnet.

Explanation: Your company has a class C network address of 200.10.57.0. You want to subdivide your physical network into 3 subnets (A, B and C) using a router as shown in the diagram at the end of the worksheet. You will need at least 20 hosts per subnet. Answer the following questions.

  1. What is the binary equivalent of the Class C network address 200.10.57.0 in the exercise?  
    __________ . __________ . __________ . __________
  2. Which octet(s) represent the network portion and which octet(s) represent the host portion of this Class C network address? 


  3. How many bits must you borrow from the host portion of the network address in order to provide at least 3 subnets and at least 20 hosts per subnet? 


  4. What will the Subnet Mask be (using dotted decimal notation) based on the number of bits borrowed in step 3? 
    _________ . _________ . __________  . __________
  5. What is the binary equivalent of the subnet mask above:
    _________ . _________ . __________  . __________

Step 3 – Class C network address with 3 subnets.

Task: Complete the table below according to the instructions. Use the information in the table to answer the questions and complete the diagram at the end of this lab.

Explanation: Be sure to specify all four octets for subnet address and subnet mask. The same subnet mask should be used for all hosts, router interfaces and all subnets. Having a common subnet mask will allow hosts and routers to determine which subnet the IP packet is intended for. Router interfaces will usually be numbered first when assigning IP addresses and hosts will receive higher numbers.

  1. Fill in the following table for each of the possible subnets that can be created by borrowing 3 bits for subnets from the fourth octet (host octet). Identify the Network Address, the Subnet Mask, the Subnetwork Address, the range of possible host IP addresses for each subnet, the broadcast address of each subnet and also indicate whether the subnet is useable or not. You will only use 3 of these subnets for the exercise.
SN#  Network Address Subnet mask Subnetwork Address Range of possible Host IP Addresses  Broadcast Address Use?
1st              
2nd             
3rd             
4th             
5th            
6th            
7th            
8th             
  1. Assign an IP Address and Subnet Mask to router interface A and write it down here.  
    __________________ / ___________________ 
  2. Assign an IP Address and Subnet Mask to router interface B and write it down here. 
    __________________ / ___________________
  3. Assign an IP Address and Subnet Mask to router interface C and write it down here. 
    __________________ / ___________________
  4. Assign a host IP Address to Host X on Subnet A and assign an IP address to Host Z on Subnet C (answers may vary). Describe the steps (using ANDing) for the process of sending an IP packet from Host X to host Z through the router. Remember, when ANDing, two 1s together the result is a 1, ANDing any other combination (1 and 0, 0 and 1 or 0 and 0) results in a Zero (0). Also, when ANDing two network IP addresses together the result of the ANDing process will be the network (or subnetwork) address of the destination IP address in the packet. Use the information from the diagram above and prior lab to help assign IP addresses and subnet masks.

  5. What is the result of the ANDing process for Host X?
    Decimal Host X IP addr: _______ . _______ . ________ . ________
    Binary Host X IP addr: _______ . _______ . ________ . ________
    Binary Subnet Mask: _______. _______ . ________ . ________
    Binary ANDing Result: _______ . _______ . ________ . ________
    Decimal ANDing Result: ________ . ________ . ________ . ________
  6. What is the result of the ANDing process for Host Z?
    Decimal Host Z IP addr: ________ . ________ . ________ . ________
    Binary Host Z IP addr: ________ . ________ . ________ . _________
    Binary Subnet Mask: ________ . ________ . _________ . _________
    Binary ANDing Result: ________ . ________ . ________ . _________
    Decimal ANDing Result: ________ . ________ . ________ . _________
  7. The Decimal ANDing result from questions 6 is the network/subnet that Host X is on. The result from question 7 is the network/subnet that Host Z is on. Are Host X and Host Z on the same network/subnet? 


  8. What will Host X now do with the packet ? 


  9. Fill in the blanks in the following diagram with the correct Network and IP addresses.


LAB 10.7.6.1 – SUBNET MASK – 2 – ANSWERS

Step 2

  1. What is the binary equivalent of the class C network address 200.10.57.0 in the exercise?
    11001000     . 00001010 . 00111001 . 00000000
  2. Which octet(s) represent the network portion and which octet(s) represent the host portion of this class C network address? 
    The first three octets are the network and the 4th octet is the host.
  3. How many bits must you borrow from the host portion of the network address in order to provide at least 3 subnets and at least 20 hosts per subnet? 
    3 bits (2^3 – 2 = 6) will create 8 possible subnets of which 6 are useable (not counting the first and the last subnet).
  4. What will the Subnet Mask be (using dotted decimal notation) based on the number of bits borrowed in step 3 ? 255 . 255 . 255 . 224 (The first 3 bit are borrowed from the left side of the host address. 128 + 64 + 32 = 224)
  5. What is the binary equivalent of the subnet mask above:
    11111111 . 11111111 . 11111111 . 11100000     (The first three octets (24 bits of all 1s) mask the original network address. The first 3 bits of the host portion 4th octet are all 1s and mask the subnet number. The last 5 bits of 0s are reserved for host Ids.)

Step 3

1. Fill in the following table

SN#  Network Address Subnet mask Subnetwork Address Range of possible Host IP Addresses  Broadcast Address Use?
1st  200.10.57.0 255.255.255.224 200.10.57.0 200.10.57.1-30 200.10.57.31 N
2nd  200.10.57.0 255.255.255.224 200.10.57.32 200.10.57.33-62 200.10.57.63 Y
3rd  200.10.57.0 255.255.255.224 200.10.57.64 200.10.57.65-94 200.10.57.95 Y
4th  200.10.57.0 255.255.255.224 200.10.57.96 200.10.57.97-126 200.10.57.127 Y
5th 200.10.57.0 255.255.255.224 200.10.57.128 200.10.57.129-158 200.10.57.159 Y
6th 200.10.57.0 255.255.255.224 200.10.57.160 200.10.57.161-190 200.10.57.191 Y
7th 200.10.57.0 255.255.255.224 200.10.57.196 200.10.57.193-222 200.10.57.223 Y
8th  200.10.57.0 255.255.255.224 200.10.57.224 200.10.57.225254 200.10.57.255 N
  1. Assign an IP Address and Subnet Mask to router interface A and write it down here.
    200.10.57.33 / 255.255.255.224 (The first subnet you can use is the .32 net. The first useable IP address for a host or router interface on the .32 net is .33 since the .32 is reserved for the address of the subnet itself, the lowest numbers are reserved for router interfaces. The subnet mask will be the same for all interfaces and subnets)
  2. Assign an IP Address and Subnet Mask to router interface B and write it down here.
    200.10.57.65 / 255.255.255.224
  3. Assign an IP Address and Subnet Mask to router interface C and write it down here.
    200.10.57.97 / 255.255.255.224
  4. Assign a host IP Address to Host X on Subnet A and assign an IP address to Host Z on Subnet C (answers may vary). Describe the steps (using ANDing) in the process of sending an IP packet from Host X to host Z through the router.
    Host X = 200.10.57.34 (.33 was used for the router interface on subnet A), Host Z = 200.10.57.98 (.97 was used for the router interface on subnet C). Host X compares (ANDs) the subnet mask to its own IP address and comes up with its own network/subnet address of 200.10.57.32. It then compares the subnet mask to the IP address of the destination host (200.10.57.98) and comes up with the network/subnet address of the target network (200.10.57.96). Since the two do not match, host X must assume that the destination host is not on its network and it send the packet to its "Default Gateway" or the nearside port of the router. The router goes through the same process on its incoming interface A and determines that network 200.10.57.96 is on it's C interface. The router forwards the packet to interface C and since the Router also knows the MAC address of hosts directly attached to its interfaces such as host Z, it forwards the packet to the 200.10.57.96 network/subnet LAN and host Z picks it up.
  5. What is the result of the ANDing process for host X?
    Decimal Host X IP addr: 200 . 10 . 57 . 34
    Binary Host X IP addr: 11001000 . 00001010 . 00111001 . 00100110
    Binary Subnet Mask: 11111111 . 11111111 . 11111111 . 11100000
    Binary ANDing Result: 11001000 . 00001010 . 00111001 . 00100000
    Decimal ANDing Result:     200 . 10 . 57 . 32
  6. What is the result of the ANDing process for host Z?
    Decimal Host Z IP addr: 200 . 10 . 57 . 98
    Binary Host Z IP addr: 11001000 . 00001010 . 00111001 . 01100110
    Binary Subnet Mask: 11111111 . 11111111 . 11111111 . 11100000
    Binary ANDing Result: 11001000 . 00001010 . 00111001 . 00100000
    Decimal ANDing Result: 200 . 10 . 57 . 96
  7. The Decimal ANDing result from questions 11 is the network/subnet that Host X is on. The result from question 12 is the network/subnet that Host Z is on. Are Host X and Host Z on the same network/subnet?
    NO
  8. What will host X do with the packet ?
    Since the destination host Z is not on the Host X Local Area Network, Host X will send the packet to the "Default Gateway" which is the IP address of Interface A on the router.
  9. When the router receives the packet from Host X and compares its interface A IP address and the destination address for host Z with the subnet mask, which router interface will it send the packet out of to get the packet to Host Z?
    Interface C