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Subnetting Surasak Sanguanpong nguan@ku.ac.th cpe.ku.ac.th/~nguan

Learn the basics of subnetting, including subnet mask computation, subnet routing, and the benefits of using subnets. Examples provided.

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Subnetting Surasak Sanguanpong nguan@ku.ac.th cpe.ku.ac.th/~nguan

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  1. Subnetting Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan Last updated: 27 June 2002

  2. Topics • The Basics of Subnetting • Subnet Mask • Computing subnets and hosts • Subnet Routing • Creating a Subnet • Example of Subnetting

  3. Addressing without Subnets • A class B “Flat Network”, more than 65000 hosts • How to manage? • Performance? 172.16.1.2 172.16.1.3 172.16.2.1 172.16.254.254 172.16.0.0

  4. Addressing with Subnets 172.16.1.2 172.16.1.3 172.16.2.2 172.16.2.3 • A class B “subdivided network”, smaller groups with routers 172.16.1.0 172.16.2.0 172.16.4.2 172.16.4.3 172.16.3.2 172.16.3.3 172.16.3.0 172.16.4.0

  5. Subnetwork benefits Smaller networks are easier to manage and troubleshoot Increase the network manager's control over the address space Subdivide on IP network number is an important initial task of network managers Overall traffic is reduced, performance may improve Subnetwork

  6. Subnet Address Before Subnetting • A subnet address is created by borrowing bit from the Host ID and designated it as a Subnet ID field Network ID Host ID After Subnetting Network ID Subnet ID Host ID

  7. Define individual hosts Define physical subnetwork How to assign subnet • Each class can have different size of subnet field Network Subnet Host choose appropriate size Class A : 2 to 22 bits Class C : 2 to 6 bits Class B : 2 to 14 bits

  8. Subnet Example • Class B address such as 172.16.0.0 might use its third byte to identify subnet Subnet Network Address Address Range 172. 16. 1. 0 #1 172.16.1.1-172.16.1.254 172. 16. 2. 0 #2 172.16.2.1-172.16.2.254 172. 16. 3. 0 #3 172.16.3.1-172.16.3.254 172. 16. 254. 0 #254 172.16.254.1-172.16.254.254

  9. Example : A class B network with 24 bits mask Network ID Subnet ID Host ID 1 1111 1111 1111 1111 1111 1111 0000 0000 Set the bit covering the network and subnet ID to 1 255. 255. 255. 0. 2 zero bit are used to mask out the host number resulting the network address Subnet mask • subnet mask is a 32 bit number, use to identify a subnet subnet mask= 255.255.255.0

  10. & 255.255.255.0 172.16.4.2 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 Masking • 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 address

  11. Subnet 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 /8 1111 1111 0000 0000 0000 0000 0000 0000 255.255.192.0 /18 1111 1111 1111 1111 1100 0000 0000 0000 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 = 172.16.0.0/24

  12. Subnet 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 subnet

  13. Routing • 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 network

  14. Subnet interpretation IP Address subnet mask Interpretation 15.20.15.2 255.255.0.0 host 15.2 on subnet 15.20.0.0 130.122.34.3 255.255.255.192 host 3 on subnet 130.122.34.0 130.122.34.132 255.255.255.192 host 4th on subnet 130.122.34.128 158.108.2.71 255.255.255.0 host 71 on subnet 158.108.2.0 200.190.155.66 255.255.255.192 host 2nd on subnet 200.190.155.64

  15. Default 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 0000

  16. Range 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 example

  17. Binary mask Octet value 1 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1 1 0 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 1 0 0 0 1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 128 192 224 240 248 252 254 255 128 64 32 16 8 4 2 1 Computing subnet mask Decimal equivalents of bit patterns

  18. 1111 1111 1111 1111 1111 1111 0000 0000 Network ID Subnet ID Host ID 255. 255. 255. 0. Compute Net and host • How many subnet and host are there with 172.16.0.0/24 8 bit subnet ID = 28=256 => 254 subnets 8 bit host ID = 28=256 => 254 hosts per subnet

  19. Network and Host relationship • Sample class C

  20. Subnetting Special Addresses Reserved 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 172.16.2.0

  21. Subnet Net Block Diagram No subnetting 2 bits • Block diagram subnetting class C 3 bits 4 bits Network Address Broadcast Address

  22. Contiguous and Noncontiguous mask no intermedite 0 gaps in the subnet mask Noncontiguous leads to complex subnetting and routing Contiguous subnet mask 1111 1111 1111 1111 1111 1111 0000 0000 intermedite 0 gaps in the subnet mask Noncontiguous subnet mask 1111 1111 1111 1111 0001 1111 0000 0000 It is strongly recommend to use contiguous subnet mask

  23. Subnet 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

  24. Example : 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 10.2.0.0 10.2.0.1 10.2.255.254 10.2.255.255 #2 Network Address Broadcast Address #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

  25. Class A Subnet with router 10.0.0.0/16 254 subnets each with 65534 hosts 10.1.0.0 10.1.0.1 to 10.1.255.254 #1 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 10.255.255.254

  26. Subnet 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 hosts

  27. Example : 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 #2 172.16.2.0 172.16.2.1 172.16.2.254 172.16.2.255 Network Address Broadcast Address #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

  28. Class B Subnet with router 172.16.1.0 172.16.1.1 to 172.16.1.254 172.16.1.0/24 254 subnets each with 65534 hosts #1 172.16.2.0 172.16.2.1 to 172.16.2.254 #2 #3 172.16.3.0 172.16.3.1 to 172.16.3.254 172.16.254.0 172.16.254.1 to 172.16.254.254 #254

  29. Subnet 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

  30. Example : 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 #2 192.68.0.64 192.68.0.65 192.68.0.94 192.68.0.95 Network Address Broadcast Address #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

  31. Class C Subnet with router 192.68.0.0/27 6 subnets each with 30 hosts 192.68.0.32 192.68.0.33 to 192.68.0.62 #1 192.68.0.64 192.68.0.65 to 192.68.0.94 #2 #3 192.68.0.96 192.68.0.97 to 192.68.0.126 192.68.0.192 192.68.0.193 to 192.68.0.222 #6

  32. Subnet 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 =??

  33. Subnet 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 =??

  34. Type 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) • the subnets may use different subnet masks • pros: utilize address spaces • cons: required well-management Variable Lengh Subnetting

  35. Problem of Static subnetting 192.68.0.32/27 used 20 hosts, waste 10 hosts • Inefficient allocation of the address space 192.68.0.64/27 used 20 hosts, waste 10 hosts 192.68.0.96/27 used 25 hosts, waste 5 hosts 192.68.0.128/27 used 25 hosts, waste 5 hosts 192.68.0.192/27 used 10 hosts, waste 20 hosts 192.68.0.224/27 used 10 hosts, waste 20 hosts

  36. Variable-Length Subnetting 192.68.0.32/27 used 20 hosts, waste 10 hosts • General Idea of VLSM • A small subnet with only a few hosts needs a subnet mask that accommodate only few hosts • A subnet with many hosts need a subnet mask to accommodate the large number of hosts 192.68.0.64/27 used 20 hosts,waste 10 hosts 192.68.0.96/27 used 25 hosts,waste 5 hosts 192.68.0.128/27 used 25 hosts,waste 5 hosts 192.68.0.192/28 used 10 hosts,waste 4 hosts 192.68.0.208/28 unused subnet Available 14 hosts 192.68.0.224/28 used 10 hosts,waste 4 hosts 192.68.0.240/28 unused subnet Available 14 hosts

  37. VLSM - An Example • three different VLSM of 172.16.0.0 CPC RDI 255.255.255.0 255.255.255.0 255.255.255.252 255.255.255.252 255.255.255.252 point-to-point link CPE 255.255.255.252 255.255.255.192

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