This document discusses IP addressing and subnetting. It begins with an overview of classful addressing and then demonstrates subnetting for class A, B, and C networks. The bulk of the document works through a practical example of VLSM subnetting to allocate IP addresses for a network topology with varying subnet size requirements. Various subnetting options are considered and a solution is presented using CIDR notation to specify the subnet mask for each allocation.

1. IP Addressing/Subnetting
Simplified
Brian Sterck
Uploaded By :

2. Class A
Subnetting
Options

3. Class B
Subnetting
Options

4. Class C
Subnetting
Options

5. Brain Benders

6. More Tools…

7. Practical Example
VLSM Exercise 1
192.168.20.0 /24
Internet
14 Nodes
17 Nodes
14 Nodes
10 Nodes
6 Nodes
23 Nodes
19 Nodes

8. Do we have enough addresses?
• How many addresses have been
allocated?
• How many are needed?

9. Address Allocation
• Total Address Needed:
LANs = 14+10+19+23+6+17+14 = 103
WANs = 4+4+4+4+4+4 = 24
Total = 103 + 24 = 127

10. Understanding IP Requirements
• Subnetworks on the LAN/WAN need to be
issued using perfect powers of 2
• 22 = 4
• 23 = 8
• 24 = 16
• 25 = 32
• 26 = 64
• 27 = 128

11. Scaling Subnets
• With a requirement of 23 nodes…
• The next perfect power of 2 that meets this
need = 32
• 25 = 32 5 host bits needed
• Last Octet is then represented as:
– A.B.C.NNNHHHHH N=Network H=Host
– 11111111.11111111.11111111.11100000

12. Subnetting Examples
SubnetMask SubnetMask # Hosts
255.255.255.0 /24 256 (254)
255.255.255.128 /25 128 (126)
255.255.255.192 /26 64 (62)
255.255.255.224 /27 32 (30)
255.255.255.240 /28 16 (14)
255.255.255.248 /29 8 (6)
255.255.255.252 /30 4 (2)

13. Practical Example
VLSM Exercise 1
192.168.20.0 /24
Internet
/28
14 Nodes
/27
/30 17 Nodes
/30
14 Nodes
/28
/30 /30 /30
10 Nodes
/30 6 Nodes
/28
/29
23 Nodes
/27
19 Nodes
/27

14. Address Allocation
• Total Addresses Needed:
LANs = 16+16+32+32+8+32+16 = 152
= /28 + /28 + /27 + /27 + /29 + /27 + /28 = 152
WANs = 4+4+4+4+4+4 = 24
= /30 + /30 + /30 + /30 + /30 + /30 = 24
Total = 103 + 52 = 157

15. Reference Points
• Use numbers, symbols, letters to represent
each LAN / WAN link
• Use some consistent method for assignment
– Size of Subnetwork 1
– Location
– Zone or Distribution Point A1

16. Assign Reference Points
VLSM Exercise 1
192.168.20.0 /24
5 /28 Internet
14 Nodes
/27 3
/30 17 Nodes
/30 10
14 Nodes
12
4 /28 8 13
11
/30 /30 /30
10 Nodes
/30 6 Nodes
6 /28 9
/29 7
23 Nodes
/27 1
19 Nodes
/27
2

17. Subnetting Options
/24
/25 /25
/26 /26 /26 /26
/27 /27 /27 /27 /27 /27 /27 /27
/28 /28
/29 /29 /29 /29
/30 /30 /30 /30 /30 /30 /30 /30

18. Subnetting Options (cont.)
/24
/25 /25
/26 /26 /26 /26
/27 /27 /27 /27 /27 /27 /27 /27
/30 /30 /30 /30 /30 /30 /30 /30

19. VLSM and Routing Protocols
• Does your routing protocol support varying
the length of the subnet mask from one
interface to another?
• Can you re-subnet a subnet differently
from one interface to another and still
advertise that subnetwork via
RIPv1, RIPv2, OSPF, EIGRP, ISIS, BGP?

20. Routing Protocols
• Supporting VLSM
– RIP version 2
– EIGRP
– OSPF
– ISIS
• Not Supporting VLSM
– IGRP
– RIP version 1

21. Which Routing Protocols will
advertise all Networks correctly?
Internet Service Provider
Point-to-Point
PPP 512k Link
192.168.16.0 /30
.129
.2 .1 .9
Frame-Relay
Service
T1 Link .17
.18
192.168.16.8 /30
192.168.65.128 /26
ISDN Cloud
128K
.130 192.168.16.16 /30
.10
192.168.64.0 /25 192.168.64.128 /25 192.168.33.96/27
.129
.1 192.168.33.128 /27
.193
.97
.129
192.168.65.192 /26 .65
192.168.32.64 /26

22. Non-VLSM capable Routing
Protocols will work just fine!
Internet Service Provider
Point-to-Point
PPP 512k Link
192.168.16.0 /30
.129
.2 .1 .9
Frame-Relay
Service
T1 Link .17
.18
192.168.16.8 /30
192.168.65.128 /26
ISDN Cloud
128K
.130 192.168.16.16 /30
.10
192.168.64.0 /25 192.168.64.128 /25 192.168.33.96/27
.129
.1 192.168.33.128 /27
.193
.97
.129
192.168.65.192 /26 .65
192.168.32.64 /26
Same Major Network = 192.168.33.0 /24

23. VLSM Exercise 1
192.168.20.0 /24
5 /28 Internet
14 Nodes
/27 3
/30 17 Nodes
14 Nodes
/30 10
12
4 /28 8 13
11
/30 /30 /30
10 Nodes
/30 6 Nodes
6 /28 9
/29 7
23 Nodes
/27 1
19 Nodes
/27
2

24. Problem!
• The 192.168.20.0 /24 network gives us:
– 1 Network
– 254 Nodes Available for Assignment
• 13 Subnetworks Required
• Network Demands range from 4 – 32
nodes per LAN / WAN segment

25. Start Subnetting
192.168.20.0 255.255.255.0
00000000

26. Subnetting
192.168.20.0 255.255.255.0
00000000
3
2 =8
5
Largest Network required = /27
32 Nodes
2 = 32

27. Subnetting
192.168.20.0 255.255.255.0
00000000 3
2 =8
Net Host
5
Largest Network required = /27
32 Nodes
2 = 32

28. Box Diagram Alternate
192.168.20.0 /24
.0
.255

29. Address Range
192.168.20.0 /24
.0
.255

30. 192.168.20.0 /24
.0
.127
.128
.255

31. 192.168.20.0 /24
.0
.127
.128
.255

32. /24
/25 /25

33. 192.168.20.0 /24
.0
/25
126
Nodes
.127
.128
/25
126
Nodes
.255

34. 192.168.20.0 /24
.0
/25
126
Nodes
.127
.128 .192
/26 /26
62 Nodes 62 Nodes
.191 .255

35. /24
/25 /25
/26 /26

36. 192.168.20.0 /24
.0
/25
126
Nodes
.127
.128 .192
/27 /26
30 Nodes
.159
.160
/27 62 Nodes
30 Nodes .255
.191

37. 192.168.20.0 /24
.0
.127
.128 .192
.159
.160
.191 .255

38. /24
/25 /25
/26 /26
/27 /27

39. /30
9
23 Nodes
1
/27
19 Nodes
/27
2

40. 192.168.20.0 /24
.0
/25
126
Nodes
.127
.128 .192
1
/27
/26
30 Nodes .159
.160
/27 62 Nodes
30 Nodes .255
.191

41. 192.168.20.0 /24
.0
/25
126
Nodes
.127
.128 .192
1
/27
/26
30 Nodes .159
.160
2
/27
62 Nodes
30 Nodes .191 .255

42. 192.168.20.0 /24
.0
/25
126
Nodes
.127
.128 .192
/27 3
/27
1
30 Nodes 30 Nodes
.159 .223
.160 .224
2
/27 /27
30 Nodes 30 Nodes
.191 .255

43. VLSM Exercise 1
192.168.20.0 /24
Internet
5 /28
14 Nodes
/27 3
/30 17 Nodes
14 Nodes
/30 10
12
4 /28 8 13
11
/30 /30 /30
10 Nodes
/30 6 Nodes
6 /28 9
/29 7
23 Nodes
/27 1
19 Nodes
/27
2

44. 192.168.20.0 /24
.0
/25
126
Nodes
.127
.128 .192
/27 3
/27
1
30 Nodes 30 Nodes
.159 .223
.160 .224 .240
/27 /28
2
4 /28
30 Nodes 14 Nodes
.191 .239 .255

45. 192.168.20.0 /24
.64
.0
/27
.95
/26
.96
/27
.63 .127
.128 .192
/27 3
/27
1
30 Nodes 30 Nodes
.159 .223
.160 .224 .240
/27 /28
2
4 /28
30 Nodes 14 Nodes
.191 .239 .255

46. 192.168.20.0 /24
.64 .80
.0
/28 /28
5 6
14 Nodes 14 Nodes
.79 .95
/26
.96
/27
.63 .127
.128 .192
/27 3
/27
1
30 Nodes 30 Nodes
.159 .223
.160 .224 .240
/27 /28
2
4 /28
14 Nodes
30 Nodes .191 .239 .255

47. 192.168.20.0 /24
.64 .80
.0
/28 /28
5 6
14 Nodes 14 Nodes
.79 .95
/26
.96 .112
7 /29
.103
.104
/28
/29
.63 .111 .127
.128 .192
/27 3
/27
1
30 Nodes 30 Nodes
.159 .223
.160 .224 .240
/27 /28
2
4 /28
14 Nodes
30 Nodes .191 .239 .255

48. 192.168.20.0 /24
.64 .80
.0
/28 /28
5 6
14 Nodes 14 Nodes
.79 .95
/26
.96 .112 .116
7 /29 /30 /30
.103 .115 .119
.104 .108 .120 .124
/30 /30 /30 /30
.63 .107 .111 .123 .127
.128 .192
/27 3
/27
1
30 Nodes 30 Nodes
.159 .223
.160 .224 .240
/27 /28
2
4 /28
14 Nodes
30 Nodes .191 .239 .255

49. 192.168.20.0 /24
.64 .80
.0
/28 /28
5 6
14 Nodes 14 Nodes
.79 .95
/26
.96 .112 /30 .116 /30
7 /29
.103 10 .115 11.119
.104 .108 .120 .124 /30
/30 /30 /30
.63 8 .107 9 .111 12 .123 13 .127
.128 .192
/27 3
/27
1
30 Nodes 30 Nodes
.159 .223
.160 .224 .240
/27 /28
2
4 /28
14 Nodes
30 Nodes .191 .239 .255

50. VLSM Exercise 1
192.168.20.0 /24
5 /28 Internet
14 Nodes
/27 3
/30 17 Nodes
14 Nodes
/30 10
12
4 /28 8 13
11
/30 /30 /30
10 Nodes
/30 6 Nodes
6 /28 9
/29 7
23 Nodes
/27 1
19 Nodes
/27
2

51. Future Expansion Danger
• Note the three locations with the and
• If we need to add a network printer or visitor
with laptop, the IP addressing scheme will have
to be modified.
• This results in downtime and careful migration
planning
There must be a better way!

52. 192.168.20.0 /24
.0
/25
126 Nodes
.127
.128 .192 .208
/28
1
/27
4 /28
30 Nodes .159
14 Nodes
.207 .223
.160 .224 .240
2
/27
/28 /28
30 Nodes .191 .239 .255

53. 192.168.20.0 /24
.0
/25
126 Nodes
.127
.128 .192 .208
/28
1
/27
4 /28
30 Nodes .159
14 Nodes
.207 .223
.160 .224 .240
2
/27
/28 /28
30 Nodes .191 .239 .255

54. 192.168.20.0 /24
.0
/25
126 Nodes
.127
.128 .192 .208
/28
1
/27
4 /28
30 Nodes .159
14 Nodes
.207 .223
.160 .224 /29 .240
2
/27 6
.231
.232
/28
30 Nodes /29
.191 .239 .255

55. 192.168.20.0 /24
.0
/25
126 Nodes
.127
.128 .192 .208
/28
1
/27
4 /28
30 Nodes .159
14 Nodes
.207 .223
.160 .224 /29 .240
6
2
/27 .231
.232
/28
/29
30 Nodes .191 .239 .255

56. 192.168.20.0 /24
.0 .64
/27
.95
/26
.96
/27
3
.63
30 Nodes .127
.128 .192 .208
/28
1
/27
4 /28
30 Nodes .159
14 Nodes
.207 .223
.160 .224 /29 .240
6
2
/27 .231
.232
/28
/29
30 Nodes .191 .239 .255

57. 192.168.20.0 /24
.64 .80 /29
.0 7
.87
/28
.88
.79
/29 .95
/26
.96
/27
3
.63
30 Nodes .127
.128 .192 .208
/28
1
/27
4 /28
30 Nodes .159
14 Nodes
.207 .223
.160 .224 /29 .240
6
2
/27 .231
.232
/28
/29
30 Nodes .191 .239 .255

58. 192.168.20.0 /24
.64 .80 /29
.0 7
.87
/27 /28
.88
.31 .79
/29 .95
.32 .48 .96
/27
5 3
/28
14 Nodes 30 Nodes
.47 .63 .127
.128 .192 .208
/28
1
/27
4 /28
30 Nodes .159
14 Nodes
.207 .223
.160 .224 /29 .240
6
2
/27 .231
.232
/28
/29
30 Nodes .191 .239 .255

59. 192.168.20.0 /24
.64 .80 /29
.0 7
.87
/27 /28
.88
.31 .79
/29 .95
.32 .48 .96
/27
5 3
/28
14 Nodes 30 Nodes
.47 .63 .127
.128 .192 .208
/28
1
/27
4 /28
30 Nodes .159
14 Nodes
.207 .223
.160 .224 .240 .244
6 /29
2
/27 .231
8 9 .247
.243
.232 .248
/29 /29
30 Nodes .191 .239 .255

60. 192.168.20.0 /24
.0 .4 .64 .68 .80 /29
10 7
13 /30
.3 .7 .71 .87
/28 .67
.8 .12 .72 .88
11 12 /29
.15 .31 .79
/29 .95
.11
.32 .48 .96
/27
5 3
/28
14 Nodes 30 Nodes
.47 .63 .127
.128 .192 .208
/28
1
/27
4 /28
30 Nodes .159
14 Nodes
.207 .223
.160 .224 .240 .244
6 /29
2
/27 .231
8 9 .247
.243
.232 .248
/29 /29
30 Nodes .191 .239 .255

61. VLSM Exercise 1
192.168.20.0 /24
5 /28 Internet
14 Nodes
/27 3
/30 17 Nodes
14 Nodes
/30 10
12
4 /28 8 13
11
/30 /30 /30
10 Nodes
/30 6 Nodes
6 /28 9
/29 7
23 Nodes
Summary Address /27 1
192.168.20.128 /25
19 Nodes
/27
2

62. VLSM Exercise 1
192.168.20.0 /24
5 /28 Internet
14 Nodes
/27 3
/30 17 Nodes
14 Nodes
/30 10
12
4 /28 8 13
11
/30 /30 /30
10 Nodes
/30 6 Nodes
6 /28 9
/29 7
23 Nodes
Summary Address /27 1
192.168.20.128 /25
19 Nodes Summary Address
/27 192.168.20.64 /26
2