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Subnetting

Subnetting. John McGarvey LYIT. Index. IP addressing classes Subnet masking Subnetting Exercises ANDing function. Intro Subnetting. IP addressing Classes within IP Class A - 16,777,214 (2 24 - 2) - Governments - start with 0

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Subnetting

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  1. Subnetting John McGarvey LYIT

  2. Index • IP addressing classes • Subnet masking • Subnetting • Exercises • ANDing function

  3. Intro Subnetting • IP addressing • Classes within IP • Class A - 16,777,214 (224 - 2) - Governments - start with 0 • Class B - 65,534(216 -2) - Medium-size companies - start with 10 • Class C - 254 (28 -2) - all other entities - start with 110 • Class D - reserved for use by Multicast services provided by The internet Group Management Group (IGMP) - start with 1110 • Multicast broadcasts allow IP datagrams to be sent to specific groups that belong to a multicast group • Class E- Not in use or only used for testing and development Reserved for future use - start with 11110

  4. IP Addressing 193.32.2.0 • We need to use bits from the host part of an IP address to create subnets • The number of bits depend on the number of subnets needed for that company Network Host

  5. Subnet Mask • Properties of the Subnet mask are • 32 bits • in binary format • expressed as 4 octets • Defined from left to right • Used by internetworking devices to find out what type of network/ subnetwork is installed/used by a company

  6. Subnet Mask • “ The most misunderstood part of IP addressing” • DO not introduce above in a class ! • Subnet Masks are used to make it easy to distinguish between network and host address • example a class B network not subnetted • 130.12.0.0 - IP address • 255.255.0.0 - Subnet mask • N NH H All network bits assigned to 1 • 11111111.11111111.0.0

  7. Subnet Mask • Default Subnet masks • Class A - 255.0.0.0 • Class B - 255.255.0.0 • Class C - 255.255.255.0 • What happens when we use subnetting?

  8. Subnetting • Why ? - Because using natural address masks are not efficient - too large • Reduces the size of the network, which in turn makes it easier to manage and troubleshoot • Breaking up the address space into useable subnets • Process involves borrowing bits from the host part of the address to create a larger network address • Subnetting is done by network administrator

  9. Subnetting Exercise A • Example of Subnetting • The Network Administrator of LYIT receives new Network address 132.8.0.0 • LYIT has 8 departments • A number of questions that need to be answered are:- • 1. What class is the network address? • 2. How many subnets are needed? • 3. How many bits are borrowed from the host part of the address? • 4. How many subnets are defined and how many of these are useable? • 5. What is the default subnet mask? • 6. What is the new customised subnet mask? • 7. What is the decimal value of each subnet? • 8. What is the total number of hosts?

  10. Subnetting • 1. What class is the network address? Ans. It’s a Class B address • 2. How many subnets are needed? Ans. 8 departments so we need 8 subnets • 3.How many bits need to be borrowed? Ans. 4 bits to provide 14 subnets • 4. How many subnets are defined and how many of these are useable? Ans. 2 x 2 x 2 x 2 = 16 are defined 16 - 2 = 14 are useable • 5. What is the default subnet mask? Ans. 255.255.0.0 or 11111111.11111111.0.0

  11. Subnetting • 6. What is the new customised subnet mask? Ans. 128+64+32+16 = 1111 starting with the leftmost significant bit value of the leftmost host octet 255.255.240.0 or 11111111.11111111.11110000.00000000 • 7. What is the decimal value of each subnet? Ans. 138.8.0.0 10001010.00001000.00010000.00000001 to and including 10001010.00001000.11100000.11111110 138.8.16.1 - 138.8.31.254 - subnet 1 138.8.32.1 - 138.8.47.254 - subnet 2 138.8.48.1 - 138.8.63.254 - subnet 3 etc 138.8.208.1 - 138.8.223.254 - subnet 14 Network, Subnet and Host

  12. Subnetting • 8. What is the total number of hosts? 14 subnets The remainder of the host part of the address is 12 bits we have 16-4 borrowed = 12 bits left for hosts each subnet can have 2 to the power of 16 -2 hosts • 212 -2 = 4094 • -2 for network and broadcast addresses • 14 useable subnets x 4094 useable host addresses • Ans. 57316

  13. Subnetting Exercise B • What would be the case if the Network Administrator were given the following network address 192.12.8.0, and the number of departments were 7. • 1. What class is the network address? • 2. How many subnets are needed? • 3. How many bits are borrowed from the host part of the address? • 4. How many subnets are defined and how many of these are useable? • 5. What is the default subnet mask? • 6. What is the new customised subnet mask? • 7. What is the decimal value of each subnet? • 8. What is the total number of hosts?

  14. Subnetting • 1. What class is the network address? • Class C as it is above 191 • 2. How many subnets are needed? • 7 subnets -why not use just 3 bits • 3. How many bits are borrowed from the host part of the address? • 4 bits are needed • 4. How many subnets are defined and how many of these are useable? • 2 x 2 x 2 x 2 = 16 are defined - 16 - 2 = 14 are useable • 5. What is the default subnet mask? • 255.255.255.0 class C • 6. What is the new customised subnet mask? • 255.255.255.240

  15. Subnetting • 7. What is the decimal value of each subnet? • 192.12.8.0 • 11000000.0001100.00001000.00010000 • to and including • 11000000.0001100.00001000.11111110 • 192.12.8.16 - 192.12.8.31 - subnet 1 • 192.12.8.32 - 192.12.8.47 - subnet 2 • 192.12.8.48 - 192.12.8.63 - subnet 3 • etc • 192.12.8.208 - 192.8.223 - subnet 14 8. What is the total number of hosts? • 14 x (24 -2 = 14) • 14 x 14 = 196 Network, Subnet and Host

  16. ANDing function • In order to route a data packet, the router must first determine the destination network/subnet address by performing a logical AND using the destination host's IP address and the subnet mask. The result will be the network/subnet address.

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