1. Chapter Subnets in TCP/IP Networks © N. Ganesan, Ph.D.

2. © N. Ganesan, All rights reserved. Chapter Objectives

3. Module Introduction to Subnetting

4. © N. Ganesan, All rights reserved. Subnetting Division of a network into subnets –For example, division of a Class B address into several Class C addresses Some of the host IDs are used for creating subnet IDs

5. © N. Ganesan, All rights reserved. Need for Subnetting Classes A and B have a large number of hosts corresponding to each network ID It may be desirable to subdivide the hosts in Class C subnets Often, there is a limitation on the number of hosts that could be hosted on a single network segment –The limitation may be imposed by concerns related to the management of hardware Smaller broadcast domains are more efficient and easy to manage

6. © N. Ganesan, All rights reserved. Subnetting Principle Use parts of the host IDs for subnetting purpose A subnet mask is used to facilitate the flow of traffic between the different subnets and the outside network (hops) –A hop is the distance a data packet travels form one node to the other

7. © N. Ganesan, All rights reserved. Using Host IDs to Subnet 1401500 Class B Network 1401510 1401520 Subnet 1 Subnet 2 Third octet is now used for subnet IDs 1401530 Subnet 3

8. © N. Ganesan, All rights reserved. Subnet Configuration 1401510 1401511 Subnet ID First Host ID 140151254 ….. Last Host ID

9. © N. Ganesan, All rights reserved. Routing of Traffic 140.15.0.0 140.15.2.0 140.15.1.0 140.15.3.0 Routing Subnets 1 2 3 Outside world

10. © N. Ganesan, All rights reserved. End of Module

11. Module Subnetting Example

12. © N. Ganesan, All rights reserved. Subnetting Example Consider the case of a class C address 195. 175.25.0 assigned to an organization Subnets can be constructed by allocating part of the higher-order bits of the host ID Assume that three of the higher-order bits of the host ID are to be reserved for that purpose

13. © N. Ganesan, All rights reserved. Subnetting Structure 195175250 11100000 Subnet Mask

14. © N. Ganesan, All rights reserved. Sub Net Last Octet Subnet ID 1 00000000 195.175.25.0 2 00100000 195.175.25.32 3 01000000 195.175.25.64 4 01100000 195.175.25.96 5 10000000 195.175.25.128 6 10100000 195.175.25.160 7 11000000 195.175.25.192 8 11100000 195.175.25.224 Usable Subnets (6)

15. © N. Ganesan, All rights reserved. Sample Subnet Division Router 195.175.25.32195.175.25.64 195.175.25.33. 195.175.25.62 195.175.25.65. 195.175.25.94 30 hosts per subnet. Subnet 2Subnet 1

16. © N. Ganesan, All rights reserved. Total Number of Subnets and Hosts All zeros and ones are not used –This has been overcome in the new RFC Total number of subnets is 6 Number of hosts per subnet is 30 Subnet mask is 255.255.255.224 –255.255.255.11100000

17. © N. Ganesan, All rights reserved. End of Module

18. Module The Routing Process

19. © N. Ganesan, All rights reserved. Overview of the Masking Process IP address and subnet masks are used for the masking operation The purpose of masking is to identify whether an IP address corresponds to a local host or a remote host The mathematical technique used is known as the ANDing process

20. © N. Ganesan, All rights reserved. ANDing Process Similar to the AND Boolean operator Consider A = B and C –A is true only when B and C are true –Otherwise, A is false for all other scenarios

21. © N. Ganesan, All rights reserved. ANDing Table BCB AND C 000 010 100 111

22. © N. Ganesan, All rights reserved. Subnet Masking AND host IP and subnet mask value at startup to identify network ID AND destination IP address and subnet mask value determine either of the following: –IP represents local host –IP represents remote host

23. © N. Ganesan, All rights reserved. Subnet Masking Example Subnet ID: 195.175.25.32 Subnet Mask: 255.255.255.224 Host address –195.175.25.34 Case 1 destination address –195.175.25.40 Case 2 destination address –195.175.25.67

24. © N. Ganesan, All rights reserved. Network Scenario Router Subnet Mask: 255.255.255.224 Host 195.175.25.34 Local Host 195.175.25.40 Outside World 195.175.25.40 195.175.25.67

25. © N. Ganesan, All rights reserved. Computing Subnet ID at Startup Host ID1951752534 11000011101011110001100100100010 Subnet Mask 255 224 11111111 11100000 ANDing Result 1951752532 11000011101011110001100100100000 Yields subnet ID.

26. © N. Ganesan, All rights reserved. TCP/IP Properties of the Host

27. © N. Ganesan, All rights reserved. Masking of Destination Address:Case 1 Destinati- nation IP 1951752540 11000011101011110001100100101000 Subnet Mask 255 224 11111111 11100000 ANDing Result 1951752532 11000011101011110001100100100000 Yields subnet ID to be that of the local subnet.

28. © N. Ganesan, All rights reserved. Case 1 Forwarding of Data Packets The destination host is local Broadcast for the hardware address of the local host at IP 195.175.25.40 Send information to the local host

29. © N. Ganesan, All rights reserved. Masking of Destination Address:Case 2 Destinati- nation IP 1951752567 11000011101011110001100101000011 Subnet Mask 255 224 11111111 11100000 ANDing Result 1951752564 11000011101011110001100101000000 Yields subnet ID to be that of different subnet.

30. © N. Ganesan, All rights reserved. Case 2 Forwarding of Data Packets The destination host is remote Send information to the gateway The router at the gateway will route the data packet to the appropriate subnet

31. © N. Ganesan, All rights reserved. Gateway IP address specified In TCP/IP properties.

32. © N. Ganesan, All rights reserved. Summary of Transmission and Routing of Data Packets Router Subnet Mask: 255.255.255.224 Host 195.175.25.34 Local Host 195.175.25.40 Subnet at 195.175.25.64 195.175.25.40 (Case 1) 195.175.25.67 (Case 2)

33. © N. Ganesan, All rights reserved. Valid Subnet Masks for Class C Addresses Subnet MaskSubnetsHostsHost Total 255.255.255.192262124 255.255.255.224630180 255.255.255.24014 196 255.255.255.248306180 255.255.255.252622124 255.255.255.254 255.255.255.255

34. © N. Ganesan, All rights reserved. End of Module

35. Module Subnetting Convention

36. © N. Ganesan, All rights reserved. Subnet Convention Consider the following Class C example –195.175.25.0/27 In the above case, the first three octets and the first three higher-order bits of the fourth octet are used in subnet masking –3*8+3 = 27 bits from the beginning of the 32 bit IP address

37. © N. Ganesan, All rights reserved. Subnet Convention Illustrated 11111111 11100000 8 Bits 3 Bits Total number of masking bits = 27 195.175.25.0/27 Network ID

38. © N. Ganesan, All rights reserved. Variable Length Subnets Source: Microsoft White Paper 135.41.0.0/16 135.41.0.0/17 135.41.128.0.0/17 135.41.128.0/21 135.41.128.0./17 135.41.248.0/21 135.41.248.0/24 1 Network, 32,766 hosts 15 Networks, 2046 hosts per network 135.41.255.0/21 Subnet 8 Networks, 254 hosts per network

39. © N. Ganesan, All rights reserved. End of Module

40. Module Classless Inter-Domain Routing (CIDR)

41. © N. Ganesan, All rights reserved. Classless Inter-Domain Routing (CIDR) To avoid the depletion of the class B addresses, it is subnetted and assigned as class C addresses To avoid the proliferation of network IDs that would complicate entries in the routing tables, they were folded for easing the routing process The above is known as CIDR

42. © N. Ganesan, All rights reserved. Subnetting of Class B Example Consider the requirement of 2000 hosts by a company Allocation of one class B network ID would yield 65,534 hosts –Far more than required The solution is to subnet a B class address –8 C class network IDs with each network being able to support 254 hosts –The total number of hosts supported is 2,032

43. © N. Ganesan, All rights reserved. CIDR CIDR enables the folding of network IDs The Internet router tables will need one entry for network ID with the use of a subnet mask for supernetting –Otherwise, the table need to carry 8 entries in the previous example RIP for IP version 2, OSPF and BGPv2 are protocols that support CIDR

44. © N. Ganesan, All rights reserved. Classless Addressing FixedVariable Fixed + Zeros Fixed + Variables Network ID Host IDs

45. © N. Ganesan, All rights reserved. End of Module

46. Module Supernetting

47. © N. Ganesan, All rights reserved. Supernetting and CIDR 220.78.168.0 Source: Microsoft White Paper on TCP/IP Network ID 220.78.168.0 Subnet Mask 255.255.248.0 (For supernetting) 220.78.175.0 220.778.168.0 Internet Router Entry Network ID...... 8 Network IDs

48. End of Module End of Chapter