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