Subnetting Example 1. 2. 3. 4.

The Development department has 74 computers The Production department has 52 computers The Administrative department has 28 computers. Each department is has a router that has 2 link addresses

In this example you are required to work out a suitable set of subnet addresses for the above network. There are three departments – each requiring different numbers of computers. Your job will be to define what the subnets will be for each department as well as subnets for each of the routers.

We can summarise and rank the different segment requirements as follows:

Subnet

Segment

Hosts

1 2 3 4 5 6

Development Production Administrative Wan link 1 Wan link 2 Wan link 3

74 52 28 2 2 2

Subnet for the Development department 128 x 2 (CIDR /25) = 64 x 4 (CIDR /26) = 32 x 8 (CIDR /27) = 16 x 16 (CIDR /28) = 8 x 32 (CIDR /29) = 4 x 64 (CIDR /30) = 2 x 128 The biggest department needs a subnet large enough for 74 hosts. We can do this by dividing the 256 network into 2 subnets each with 128 computers. We cannot divide into four segments as each subnet would have a maximum of 62 addresses – not enough. Need only 1 bit for the 2 subnets so therefore use CIDR /25 (CIDR is the number of bits used for the network plus the subnet bits) Each subnet would have a max of 128 addresses Subnet 1 (for the Development Department) Subnet 2 Network ID 192.168.1.0 192.168.1.128 First host 192.168.1.1 192.168.1.129 Last host 192.168.1.126 192.168.1.254 Broadcast address 192.168.1.127 192.168.1.255 Subnet mask 255.255.255.128 The only addresses available to other departments are between 192.168.1.128 and 192.168.1.255

Subnet for the production department 128 x 2 (CIDR /25) = 64 x 4 (CIDR /26) = 32 x 8 (CIDR /27) = 16 x 16 (CIDR /28) = 8 x 32 (CIDR /29) = 4 x 64 (CIDR /30) = 2 x 128 Production department requires a subnet of 52 computers. Note that 12 addresses will be unused. /26 networks always have subnet mask 255.255.255.192 – see the table on the last page. 256 / 4 = 64 hosts. So make a table that has addresses for 4 subnets Network ID First Host Last Host Broadcast Address Subnet Mask CIDR 26

Subnet 1 192.168.1.0 192.168.1.1 192.168.1.162 192.168.1.63 255.255.255.192

Subnet 2 192.168.1.64 192.168.1.65 192.168.1.126 192.168.1.127 255.255.255.192

Subnet 3 192.168.1.128 192.168.1.65 192.168.1.190 192.168.1.191 255.255.255.192

Subnet 4 192.168.1.192 192.168.1.193 192.168.1.254 192.168.1.255 255.255.255.192

Subnet used by Development

Subnet used by Development

Production Department.

No need to use this subnet

The network ID for the Production department is 192.168.1.128. The subnet mask is 192.168.1.192 and the CIDR is 26

Subnet for the Administration Department There are 28 hosts required in the Administration Department subnet. This means we need to divide the network into 8 subnets, each of size 32 addresses. 128 x 2 (CIDR 25) = 64 x 4 (CIDR 26) = 32 x 8 (CIDR 27) = 16 x 16 (CIDR 28) = 8 x 32 (CIDR 29) = 4 x 64 (CIDR 30) = 2 x 128 The subnet to use for any and all /27 subnets is 255.255.255.224

Subnet Network ID First Host Last Host Broadcast Address Subnet Mask CIDR 27

1 0 1

2 32

3 64

4 96

5 128

6 160

31

63

95

127

159

191

Dev

Dev

Dev

Dev

Prod

Prod

7 8 192.168.1.192 224 192.168.1.193 192.168.1.222 192.168.1.223 255 255.255.255.224 Admin

WAN link 1 requires a subnet of 4 addresses. 128 x 2 (CIDR /25) = 64 x 4 (CIDR /26) = 32 x 8 (CIDR /27) = 16 x 16 (CIDR /28) = 8 x 32 (CIDR /29) = 4 x 64 (CIDR /30) = 2 x 128 NOTE that the subnet mask will be 255.255.255.252 when we divide a class c network into 64 subnets – see the table at the end. Each of the WAN routers needs two IP addresses plus a broadcast IP address and a network IP address (4 addresses altogether). Therefore the subnets for the routers will come from 192.168.1.0/30. There are 64 subnets for this network. We need to focus on choosing subnets of IP addresses that have not been used for other subnets. 1 0

33 128

34 132

35 136

2 4

36 140

3 8

4 12

37 144

5 16

38 148

6 20

39 152

7 24

40 156

8 28

41 160

9 32

10 36

42 164

11 40

43 168

12 44

44 172

13 48

45 176

14 52

46 180

15 56

16 60

47 184

17 64

48 188

18 68

19 72

49 192

50 196

WAN link 1 requires two hosts and the first available subnet for this is subnet 57.

20 76

51 200

21 80

22 84

52 204

23 88

53 208

24 92

25 96

54 212

55 216

26 100

56 220

27 104

57 224 225 226 227

28 108

58 228 229 230 231

29 112

59 232 233 234 235

30 116

60 236

31 120

61 240

32 124

62 244

63 248

64 252

Subnet Network ID First Host Last Host Broadcast Address Subnet Mask CIDR

57 of 64 192.168.1.224 192.168.1.225 192.168.1.226 192.168.1.227 255.255.255.252 30

NOTE subnet mask is always 252 when dividing a network into 64 segments. Use the table at the end of this tutorial to look up the Subnet mask for a particular segmentation of a network: WAN link 2 requires two hosts and the first available subnet for this is subnet 58. Subnet Network ID First Host Last Host Broadcast Address Subnet Mask CIDR

58 of 64 192.168.1.228 192.168.1.229 192.168.1.230 192.168.1.231 255.255.255.252 30

WAN link 3 is subnet 59 Subnet Network ID First Host Last Host Broadcast Address Subnet Mask CIDR

58 of 64 192.168.1.232 192.168.1.233 192.168.1.234 192.168.1.235 255.255.255.252 30

Subnet Masks for different sizes of network segment CIDR Total number Network Description: Notation: of addresses: Mask: -------------------------------------------------------------/0 4,294,967,296 0.0.0.0 Every Address /1 2,147,483,648 128.0.0.0 128 /8 nets /2 1,073,741,824 192.0.0.0 64 /8 nets /3 536,870,912 224.0.0.0 32 /8 nets /4 268,435,456 240.0.0.0 16 /8 nets /5 134,217,728 248.0.0.0 8 /8 nets /6 67,108,864 252.0.0.0 4 /8 nets /7 33,554,432 254.0.0.0 2 /8 nets /8 16,777,214 255.0.0.0 1 /8 net -------------------------------------------------------------/9 8,388,608 255.128.0.0 128 /16 nets /10 4,194,304 255.192.0.0 64 /16 nets /11 2,097,152 255.224.0.0 32 /16 nets /12 1,048,576 255.240.0.0 16 /16 nets /13 524,288 255.248.0.0 8 /16 nets /14 262,144 255.252.0.0 4 /16 nets /15 131.072 255.254.0.0 2 /16 nets /16 65,536 255.255.0.0 1 /16 -------------------------------------------------------------/17 32,768 255.255.128.0 128 /24 nets /18 16,384 255.255.192.0 64 /24 nets /19 8,192 255.255.224.0 32 /24 nets /20 4,096 255.255.240.0 16 /24 nets /21 2,048 255.255.248.0 8 /24 nets /22 1,024 255.255.252.0 4 /24 nets /23 512 255.255.254.0 2 /24 nets /24 256 255.255.255.0 1 /24 -------------------------------------------------------------/25 128 255.255.255.128 Half of a /24 /26 64 255.255.255.192 Fourth of a /24 /27 32 255.255.255.224 Eighth of a /24 /28 16 255.255.255.240 1/16th of a /24 /29 8 255.255.255.248 5 Usable addresses /30 4 255.255.255.252 1 Usable address /31 2 255.255.255.254 Unusable /32 1 255.255.255.255 Single host