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1/37                                 Subnetting                                      Surasak Sanguanpong                  ...
3/37Addressing without Subnets        172.16.1.2 172.16.1.3         172.16.2.1           172.16.254.254                   ...
5/37 Subnetwork benefits                        Smaller networks                      are easier to manage Increase the ne...
7/37How to assign subnet     Each class can have different size of subnet field                     Define physical subnet...
9/37Subnet mask     subnet mask is a 32 bit number, use to identify a subnet                         Example : A class B n...
11/37Subnet mask in Prefix format    The number of routing bits (network and subnet bits) in each    subnet mask can also ...
13/37Routing   Hosts and routers perform logical AND to send packets         172.16.1.2      172.16.1.3                   ...
15/37Default Subnet mask    A default subnet mask : a subnet mask with no    subnetting       Class A             255.0.0....
17/37Computing subnet maskDecimal equivalents of bit patterns             Binary mask                            Octet val...
19/37Network and Host relationship    Sample class CNumber of            Number of       Number of                 Total  ...
21/37Subnet Net Block Diagram     No subnetting                  2 bits                                                   ...
23/37Subnet Class A Example       subnet mask                                             Interpretation 255.0.0.0        ...
25/37Class A Subnet with router              10.1.0.0                10.1.0.1 to                             10.0.0.0/16  ...
27/37Example : Class B Subnet Address Table                                      IP Address : 176.16.0.0 /24              ...
29/37Subnet Class C Example         subnet mask                                            Interpretation     255.255.255....
31/37Class C Subnet with router              192.68.0.32         192.68.0.33 to                             192.68.0.0/27 ...
33/37Subnet Exercise (2)  Given IP address 192.150.251, find out the following to yield  not more than 32 hosts per subnet...
35/37Problem of Static subnetting                                   192.68.0.32/27                                   used ...
37/37 VLSM - An Example     three different VLSM of 172.16.0.0                                          CPC               ...
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Subnet

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Subnet

  1. 1. 1/37 Subnetting Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan Last updated: 27 June 2002Applied Network Research Group Department of Computer Engineering, Kasetsart University 2/37Topics The Basics of Subnetting Subnet Mask Computing subnets and hosts Subnet Routing Creating a Subnet Example of SubnettingApplied Network Research Group Department of Computer Engineering, Kasetsart University
  2. 2. 3/37Addressing without Subnets 172.16.1.2 172.16.1.3 172.16.2.1 172.16.254.254 172.16.0.0 A class B “Flat Network”, more than 65000 hosts How to manage? Performance?Applied Network Research Group Department of Computer Engineering, Kasetsart University 4/37Addressing with Subnets 172.16.1.2 172.16.1.3 172.16.2.2 172.16.2.3 172.16.1.0 172.16.2.0 172.16.3.2 172.16.3.3 172.16.4.2 172.16.4.3 172.16.3.0 172.16.4.0 A class B “subdivided network”, smaller groups with routersApplied Network Research Group Department of Computer Engineering, Kasetsart University
  3. 3. 5/37 Subnetwork benefits Smaller networks are easier to manage Increase the network Overall traffic is and troubleshoot managers control overreduced, performance may improve the address space Subnetwork Subdivide on IP network number is an important initial task of network managers Applied Network Research Group Department of Computer Engineering, Kasetsart University 6/37 Subnet Address Before Subnetting Network ID Host ID After Subnetting Network ID Subnet ID Host ID A subnet address is created by borrowing bit from the Host ID and designated it as a Subnet ID field Applied Network Research Group Department of Computer Engineering, Kasetsart University
  4. 4. 7/37How to assign subnet Each class can have different size of subnet field Define physical subnetwork Define individual hosts Network Subnet Host choose appropriate size Class A : Class C : 2 to 22 bits Class B : 2 to 6 bits 2 to 14 bitsApplied Network Research Group Department of Computer Engineering, Kasetsart University 8/37Subnet Example Class B address such as 172.16.0.0 might use its third byte to identify subnetSubnet Network Address Address Range #1 172. 16. 1. 0 172.16.1.1-172.16.1.254 #2 172. 16. 2. 0 172.16.2.1-172.16.2.254 #3 172. 16. 3. 0 172.16.3.1-172.16.3.254#254 172. 16. 254. 0 172.16.254.1-172.16.254.254Applied Network Research Group Department of Computer Engineering, Kasetsart University
  5. 5. 9/37Subnet mask subnet mask is a 32 bit number, use to identify a subnet Example : A class B network with 24 bits mask Network ID Subnet ID Host ID 1 Set the bit 1111 1111 1111 1111 1111 1111 0000 0000 covering the network and subnet ID to 1 255. 255. 255. 0. subnet mask= 255.255.255.02 zero bit are used to mask out the host number resulting the network addressApplied Network Research Group Department of Computer Engineering, Kasetsart University 10/37Masking 172.16.4.2 & 255.255.255.0 1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 0 1 0 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 172.16.4.0 A “bitwise-and” between IP address and subnet mask yields a network address. Note that zeros bit are used to mask out the host number resulting the network addressApplied Network Research Group Department of Computer Engineering, Kasetsart University
  6. 6. 11/37Subnet mask in Prefix format The number of routing bits (network and subnet bits) in each subnet mask can also be indicated by the "/n " format. 255.0.0.0 1111 1111 0000 0000 0000 0000 0000 0000 /8 255.255.192.0 1111 1111 1111 1111 1100 0000 0000 0000 /18 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000 /24 255.255.255.240 1111 1111 1111 1111 1111 1111 1111 0000 /28 172.16.0.0 255.255.255.0 = 172.16.0.0/24Applied Network Research Group Department of Computer Engineering, Kasetsart University 12/37Subnet routing Traffic is routed to a host by looking “bit-wise and” results if dest_ip_addr & subnet_mask = = my_ip_addr & subnet_mask send pkt on local network %dest ip addr is on the same subnet else send pkt to router %dest ip addr is on diff subnetApplied Network Research Group Department of Computer Engineering, Kasetsart University
  7. 7. 13/37Routing Hosts and routers perform logical AND to send packets 172.16.1.2 172.16.1.3 172.16.2.2 172.16.2.3 To 172.16.4.2 1 172.16.1.0/24 172.16.2.0/24 2 172.16.4.2 172.16.4.3 172.16.3.2 172.16.3.3 3 172.16.3.0/24 172.16.4.0/24 172.16.1.3 has a packet for 172.16.4.2 and determine that it is on other subnetwork The packet is sent to the router The router performs a subnet masking and sends the packet to the destination networkApplied Network Research Group Department of Computer Engineering, Kasetsart University 14/37Subnet interpretation IP Address subnet mask Interpretation15.20.15.2 255.255.0.0 host 15.2 on subnet 15.20.0.0130.122.34.3 255.255.255.192 host 3 on subnet 130.122.34.0130.122.34.132 255.255.255.192 host 4th on subnet 130.122.34.128158.108.2.71 255.255.255.0 host 71 on subnet 158.108.2.0200.190.155.66 255.255.255.192 host 2nd on subnet 200.190.155.64Applied Network Research Group Department of Computer Engineering, Kasetsart University
  8. 8. 15/37Default Subnet mask A default subnet mask : a subnet mask with no subnetting Class A 255.0.0.0 1111 1111 0000 0000 0000 0000 0000 0000 Class B 255.255.0.0 1111 1111 1111 1111 0000 0000 0000 0000 Class C 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000Applied Network Research Group Department of Computer Engineering, Kasetsart University 16/37Range of bit A default subnet mask : a subnet mask with no subnetting IP 172 16 0 0 Default subnet 255 255 0 0 New subnet 255 255 255 0 Define a subnet mask by extending the network portion to the right, 8 bits in this exampleApplied Network Research Group Department of Computer Engineering, Kasetsart University
  9. 9. 17/37Computing subnet maskDecimal equivalents of bit patterns Binary mask Octet value128 64 32 16 8 4 2 11 0 0 0 0 0 0 0 1281 1 0 0 0 0 0 0 1921 1 1 0 0 0 0 0 2241 1 1 1 0 0 0 0 2401 1 1 1 1 0 0 0 2481 1 1 1 1 1 0 0 2521 1 1 1 1 1 1 0 2541 1 1 1 1 1 1 1 255Applied Network Research Group Department of Computer Engineering, Kasetsart University 18/37Compute Net and host How many subnet and host are there with 172.16.0.0/24 255. 255. 255. 0. 1111 1111 1111 1111 1111 1111 0000 0000 Network ID Subnet ID Host ID8 bit subnet ID = 28=256 => 254 subnets8 bit host ID = 28=256 => 254 hosts per subnetApplied Network Research Group Department of Computer Engineering, Kasetsart University
  10. 10. 19/37Network and Host relationship Sample class CNumber of Number of Number of Total Percents subnet subnets hosts number of used bits created per subnet hosts 2 2 62 124 49% 3 6 30 180 71% 4 14 14 196 77% 5 30 6 180 71% 6 62 2 124 49%Applied Network Research Group Department of Computer Engineering, Kasetsart University 20/37Subnetting Special AddressesReserved addresses that are not allowed to be assigned to any node NetID HostID Purpose any All 0s Subnetwork Address Example: 172.16.2.0/24 Subnetwork 172.16.2.0 any All 1s Subnet-directed Broadcast Example: 172.16.2.255/24 Directed broadcast of the subnetwork 172.16.2.0Applied Network Research Group Department of Computer Engineering, Kasetsart University
  11. 11. 21/37Subnet Net Block Diagram No subnetting 2 bits Block diagram subnetting class C 3 bits 4 bits Network Address Broadcast AddressApplied Network Research Group Department of Computer Engineering, Kasetsart University 22/37Contiguous and Noncontiguous mask no intermedite 0 gaps in the subnet mask Contiguous subnet mask 1111 1111 1111 1111 1111 1111 0000 0000 intermedite 0 gaps Noncontiguous subnet mask in the subnet mask 1111 1111 1111 1111 0001 1111 0000 0000 Noncontiguous leads to complex subnetting and routing It is strongly recommend to use contiguous subnet maskApplied Network Research Group Department of Computer Engineering, Kasetsart University
  12. 12. 23/37Subnet Class A Example subnet mask Interpretation 255.0.0.0 1 network with 1677214 hosts (default subnet) 255.255.0.0 254 subnets each with 65534 hosts 255.255.128.0 510 subnets each with 32768 hosts 255.255.192.0 1022 subnets each with 16382 hosts 255.255.255.0 65534 subnets each with 254 hosts Applied Network Research Group Department of Computer Engineering, Kasetsart University 24/37Example : Class A Subnet Address Table IP Address : 10.0.0.0/16 SubnetID all 0s 10.0.0.0 10.0.0.1 10.0.255.254 10.0.255.255#1 10.1.0.0 10.1.0.1 10.1.255.254 10.1.255.255 Broadcast Address Network Address#2 10.2.0.0 10.2.0.1 10.2.255.254 10.2.255.255#254 10.254.0.0 10.254.0.1 10.254.255.254 10.254.255.255 10.255.0.0 10.255.0.1 10.255.255.254 10.255.255.255 SubnetID all 1s Applied Network Research Group Department of Computer Engineering, Kasetsart University
  13. 13. 25/37Class A Subnet with router 10.1.0.0 10.1.0.1 to 10.0.0.0/16 #1 10.1.255.254 254 subnets each with 65534 hosts 10.2.0.0 10.2.0.1 to 10.2.255.254 #2 #3 10.3.0.0 10.3.0.1 to 10.3.255.254 #254 10.255.0.0 10.255.0.1 to 10.255.255.254Applied Network Research Group Department of Computer Engineering, Kasetsart University 26/37Subnet Class B Example subnet mask Interpretation 255.255.0.0 1 network with 65534 hosts (default subnet) 255.255.192.0 2 subnets each with 16382 hosts 255.255.252.0 62 subnets each with 1022 hosts 255.255.255.0 254 subnets each with 254 hosts 255.255.255.252 16382 subnets each with 2 hostsApplied Network Research Group Department of Computer Engineering, Kasetsart University
  14. 14. 27/37Example : Class B Subnet Address Table IP Address : 176.16.0.0 /24 SubnetID all 0s 172.16.0.0 172.16.0.1 172.16.0.254 172.16.0.255#1 172.16.1.0 172.16.1.1 172.16.1.254 172.16.1.255 Broadcast Address Network Address#2 172.16.2.0 172.16.2.1 172.16.2.254 172.16.2.255#254 172.16.254.0 172.16.254.1 176.16.254.254 176.16.254.255 176.16.255.0 176.16.255.1 176.16.255.254 176.16.255.255 SubnetID all 1s Applied Network Research Group Department of Computer Engineering, Kasetsart University 28/37Class B Subnet with router #1 172.16.1.0 172.16.1.1 to 172.16.1.0/24 172.16.1.254 254 subnets each 172.16.2.0 172.16.2.1 to with 65534 hosts 172.16.2.254 #2 #3 172.16.3.0 172.16.3.1 to 172.16.3.254 #254 172.16.254.0 172.16.254.1 to 172.16.254.254 Applied Network Research Group Department of Computer Engineering, Kasetsart University
  15. 15. 29/37Subnet Class C Example subnet mask Interpretation 255.255.255.0 1 network with 254 hosts (default subnet) 255.255.255.192 2 subnets each with 62 host 255.255.255.224 6 subnets each with 30 hosts 255.255.255.240 14 subnets each with 14 hosts 255.255.255.252 62 subnets each with 2 hosts Applied Network Research Group Department of Computer Engineering, Kasetsart University 30/37Example : Class C Subnet Address Table IP Address : 192.68.0.0 /27 SubnetID all 0s 192.68.0.0 192.68.0.1 192.68.0.30 192.68.0.31#1 192.68.0.32 192.68.0.33 192.68.0.62 192.68.0.63 Broadcast Address Network Address#2 192.68.0.64 192.68.0.65 192.68.0.94 192.68.0.95#6 192.68.0.192 192.68.0.193 192.68.0.222 192.68.0.223 192.68.0.224 192.68.0.225 192.68.0.254 192.68.0.255 SubnetID all 1s Applied Network Research Group Department of Computer Engineering, Kasetsart University
  16. 16. 31/37Class C Subnet with router 192.68.0.32 192.68.0.33 to 192.68.0.0/27 #1 192.68.0.62 6 subnets each 192.68.0.64 192.68.0.65 to with 30 hosts 192.68.0.94 #2 #3 192.68.0.96 192.68.0.97 to 192.68.0.126 #6 192.68.0.192 192.68.0.193 to 192.68.0.222Applied Network Research Group Department of Computer Engineering, Kasetsart University 32/37Subnet Exercise (1) Given IP address 161.200, find out the following to yield not more than 256 hosts per subnet net mask= ?? start net id =?? end net id=?? #of subnet =??Applied Network Research Group Department of Computer Engineering, Kasetsart University
  17. 17. 33/37Subnet Exercise (2) Given IP address 192.150.251, find out the following to yield not more than 32 hosts per subnet net mask= ?? start net id =?? end net id=?? #of subnet =??Applied Network Research Group Department of Computer Engineering, Kasetsart University 34/37Type of Subnetting Static Subnetting all subnets in the subnetted network use the same subnet mask pros: simply to implement, easy to maintain cons: wasted address space (consider a network of 4 hosts with 255.255.255.0 wastes 250 IP) Variable Lengh Subnetting the subnets may use different subnet masks pros: utilize address spaces cons: required well-managementApplied Network Research Group Department of Computer Engineering, Kasetsart University
  18. 18. 35/37Problem of Static subnetting 192.68.0.32/27 used 20 hosts, waste 10 hosts 192.68.0.64/27 used 20 hosts, waste 10 hosts 192.68.0.96/27 used 25 hosts, waste 5 hosts Inefficient allocation of 192.68.0.128/27 used 25 hosts, the address space waste 5 hosts 192.68.0.192/27 used 10 hosts, waste 20 hosts 192.68.0.224/27 used 10 hosts, waste 20 hostsApplied Network Research Group Department of Computer Engineering, Kasetsart University 36/37Variable-Length Subnetting 192.68.0.32/27 General Idea of VLSM used 20 hosts, General Idea of VLSM waste 10 hosts A small subnet with only a A small subnet with only a few hosts needs a subnet few hosts needs a subnet 192.68.0.64/27 used 20 hosts, mask that accommodate mask that accommodate waste 10 hosts only few hosts only few hosts 192.68.0.96/27 A subnet with many hosts A subnet with many hosts used 25 hosts, need a subnet mask to need a subnet mask to waste 5 hosts accommodate accommodate the large number of hosts the large number of hosts 192.68.0.128/27 used 25 hosts, waste 5 hosts 192.68.0.192/28 192.68.0.208/28 used 10 hosts, unused subnet waste 4 hosts Available 14 hosts 192.68.0.224/28 192.68.0.240/28 used 10 hosts, unused subnet waste 4 hosts Available 14 hostsApplied Network Research Group Department of Computer Engineering, Kasetsart University
  19. 19. 37/37 VLSM - An Example three different VLSM of 172.16.0.0 CPC RDI 255.255.255.0 255.255.255.0 255.255.255.252 255.255.255.252 255.255.255.252point-to-point link CPE 255.255.255.252 255.255.255.192 Applied Network Research Group Department of Computer Engineering, Kasetsart University

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