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# Classless subnetting

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### Classless subnetting

1. 1. Subnetting<br />Rick Graziani<br />Cabrillo College<br />Note: These example use classless addressing. Instead of a default classful mask, a network mask is given.<br />
2. 2. Rick Graziani graziani@cabrillo.edu<br />2<br />What is subnetting?<br />Network<br />Network<br />Subnet<br />Host<br />Network<br />Network<br />Host<br />Host<br />172<br />16<br />0<br />0<br />Subnetting is the process of borrowing bits from the HOST bits, in order to divide the larger network into small subnets.<br />Subnetting does NOT give you more hosts, but actually costs you hosts.<br />You lose two host IP Addresses for each subnet, and perhaps one for the subnet IP address and one for the subnet broadcast IP address.<br />You lose the last subnet and all of it’s hosts’ IP addresses as the broadcast for that subnet is the same as the broadcast for the network.<br />In older networks, you would have lost the first subnet, as the subnet IP address is the same as the network IP address. (This subnet can be used in most networks.)<br />
3. 3. Rick Graziani graziani@cabrillo.edu<br />3<br />Analogy<br />                                                               <br />                                                               <br />Dividing the barrel of apples into small barrels or baskets does not give us any more apples…<br />100 Apples<br />
4. 4. Rick Graziani graziani@cabrillo.edu<br />4<br />Analogy<br />                                                               <br />                                                               <br />                                                               <br />10 barrels x 10 apples = 100 apples<br />10<br />10<br />10<br />100 Apples (10 * 10)<br />10<br />10<br />10<br />10<br />10<br />10<br />It is the same as taking a barrel of 100 apples and dividing it into 10 barrels of 10 apples each.<br />10<br />
5. 5. Rick Graziani graziani@cabrillo.edu<br />5<br />Analogy<br />                                                               <br />                                                               <br />100 – 2 apples = 98 Usable Apples<br />Before subnetting:<br />In any network (or subnet) we can not use all the IP addresses for host addresses.<br />We lose two addresses for every network or subnet.<br />Network Address - One address is reserved to that of the network. <br />Broadcast Address – One address is reserved to address all hosts in that network or subnet. <br />98 Apples (100 – 2)<br />
6. 6. Rick Graziani graziani@cabrillo.edu<br />6<br />                                                               <br />                                                               <br />                                                               <br />10 barrels x 8 apples = 80 apples<br />8<br />8<br />8<br />(less 2)<br />(less 2)<br />(less 2)<br />8<br />8<br />8<br />80 Apples 10 * (10 - 2)<br />(less 2)<br />(less 2)<br />(less 2)<br />8<br />8<br />8<br />(less 2)<br />(less 2)<br />(less 2)<br />In subnetting we will see that we continue to lose two apples per subnet, one for the address and one for the broadcast.<br />8<br />
7. 7. Rick Graziani graziani@cabrillo.edu<br />7<br />                                                               <br />                                                               <br />                                                               <br />10 barrels x 8 apples = 80 apples<br />X<br />8<br />8<br />8<br />---<br />(less 2)<br />(less 2)<br />(less 2)<br />8<br />8<br />8<br />64 Apples 8 * (10 - 2)<br />(less 2)<br />(less 2)<br />(less 2)<br />8<br />8<br />8<br />(less 2)<br />(less 2)<br />(less 2)<br />We “might” also lose the last basket of apples, subnet, as it contains the broadcast address for the entire network.<br />In older networks, we “might” also lost the first basket, subnet, as it contained the address of the entire network, but this is usually no longer the case.<br />X<br />8<br />---<br />
8. 8. Rick Graziani graziani@cabrillo.edu<br />8<br />Subnet Example<br />Network address 172.16.0.0 with /16 network mask<br />Network<br />Network<br />Host<br />Host<br />172<br />16<br />0<br />0<br />
9. 9. Rick Graziani graziani@cabrillo.edu<br />9<br />Network<br />Network<br />Subnet<br />Host<br />Subnet Example<br />Network address 172.16.0.0 with /16 network mask<br />Network<br />Network<br />Host<br />Host<br />172<br />16<br />0<br />0<br />Using Subnets: subnet mask 255.255.255.0 or /24<br />Network Mask: 255.255.0.0 or /16<br />11111111<br />11111111<br />00000000<br />00000000<br />Subnet Mask: 255.255.255.0 or /24<br />11111111<br />11111111<br />11111111<br />00000000<br />Applying a mask which is larger than the default subnet mask, will divide your network into subnets.<br />Subnet mask used here is 255.255.255.0 or /24<br />
10. 10. Rick Graziani graziani@cabrillo.edu<br />10<br />Network<br />Network<br />Subnet<br />Host<br />172<br />16<br />0<br />Host<br />172<br />16<br />1<br />Host<br />172<br />16<br />2<br />Host<br />172<br />16<br />3<br />Host<br />172<br />16<br />Etc.<br />Host<br />172<br />16<br />254<br />Host<br />172<br />16<br />255<br />Host<br />Subnet Example<br />Network address 172.16.0.0 with /16 network mask<br />Using Subnets: subnet mask 255.255.255.0 or /24<br />Subnets<br />255 Subnets<br />28 - 1<br />Cannot use last subnet as it contains broadcast address<br />
11. 11. Rick Graziani graziani@cabrillo.edu<br />11<br />Network<br />Network<br />Subnet<br />Host<br />172<br />16<br />0<br />0<br />172<br />16<br />1<br />0<br />172<br />16<br />2<br />0<br />172<br />16<br />3<br />0<br />172<br />16<br />Etc.<br />0<br />172<br />16<br />254<br />0<br />172<br />16<br />255<br />0<br />Subnet Example<br />Network address 172.16.0.0 with /16 network mask<br />Using Subnets: subnet mask 255.255.255.0 or /24<br />Subnets Addresses<br />255 Subnets<br />28 - 1<br />Cannot use last subnet as it contains broadcast address<br />
12. 12. Rick Graziani graziani@cabrillo.edu<br />12<br />Network<br />Network<br />Subnet<br />Hosts<br />172<br />16<br />0<br />1<br />172<br />16<br />1<br />1<br />172<br />16<br />2<br />1<br />172<br />16<br />3<br />1<br />172<br />16<br />Etc.<br />1<br />172<br />16<br />254<br />1<br />172<br />16<br />255<br />Host<br />Subnet Example<br />Class B address 172.16.0.0 with /16 network mask<br />Using Subnets: subnet mask 255.255.255.0 or /24<br />Hosts Addresses<br />254<br />254<br />254<br />254<br />254<br />254<br />Each subnet has 254 hosts, 28 – 2 <br />
13. 13. Rick Graziani graziani@cabrillo.edu<br />13<br />Network<br />Network<br />Subnet<br />Host<br />172<br />16<br />0<br />255<br />172<br />16<br />1<br />255<br />172<br />16<br />2<br />255<br />172<br />16<br />3<br />255<br />172<br />16<br />Etc.<br />255<br />172<br />16<br />254<br />255<br />172<br />16<br />255<br />255<br />Subnet Example<br />Network address 172.16.0.0 with /16 network mask<br />Using Subnets: subnet mask 255.255.255.0 or /24<br />Broadcast Addresses<br />255 Subnets<br />28 - 1<br />Cannot use last subnet as it contains broadcast address<br />
14. 14. Rick Graziani graziani@cabrillo.edu<br />14<br />Subnet Example<br />Network address 172.16.0.0 with /16 network mask<br />Using Subnets: subnet mask 255.255.255.0 or /24<br />172.16.0.0/24<br />172.16.10.0/24<br />172.16.25.0/24<br />172.16.5.0/24<br />
15. 15. Rick Graziani graziani@cabrillo.edu<br />15<br />Important things to remember about Subnetting<br />You can only subnet the host portion, you do not have control of the network portion.<br />Subnetting does not give you more hosts, it only allows you to divide your larger network into smaller networks.<br />When subnetting, you will actually lose hosts:<br />For each subnet you lose the address of that subnet<br />For each subnet you lose the broadcast address of that subnet<br />You “may” lose the first and lastlast subnets (coming)<br />Analogy: Large barrel of 100 apples.<br />Why subnet?<br />Divide larger network into smaller network.<br />Limit layer 2 and layer 3 broadcasts to their subnet.<br />Better management of traffic.<br />