Ccna routing and Switching Complete Study Guide

Practice Example #4C: 255.255.255.240 (/28)

Let’s practice another one:

192.168.10.0 = Network address

255.255.255.240 = Subnet mask 

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Subnetting Basics 

149

 

■

Subnets? 240 is 11110000 in binary. 2

4

 = 16.

 

■

Hosts? 4 host bits, or 2

4

 – 2 = 14.

 

■

Valid subnets? 256 – 240 = 16. Start at 0: 0 + 16 = 16. 16 + 16 = 32. 32 + 16 = 48.  

48 + 16 = 64. 64 + 16 = 80. 80 + 16 = 96. 96 + 16 = 112. 112 + 16 = 128. 128 + 16 = 

144. 144 + 16 = 160. 160 + 16 = 176. 176 + 16 = 192. 192 + 16 = 208. 208 + 16 = 224. 

224 + 16 = 240.

 

■

Broadcast address for each subnet?

 

■

Valid hosts?

To answer the last two questions, check out the following table. It gives you the subnets, 

valid hosts, and broadcast addresses for each subnet. First, find the address of each subnet 

using the block size (increment). Second, find the broadcast address of each subnet incre-

ment, which is always the number right before the next valid subnet, and then just fill in 

the host addresses. The following table shows the available subnets, hosts, and broadcast 

addresses provided from a Class C 255.255.255.240 mask.

Subnet

First host

Last host

0

1

14

15

16

17

30

31

32

33

46

47

48

49

62

63

64

65

78

79

80

81

94

95

96

97

110

111

112

113

126

127

128

129

142

143

144

145

158

159

160

161

174

175

176

177

190

191

192

193

206

207

208

209

222

223

224

225

238

239

240

241

254

255

Broadcast

Cisco has figured out that most people cannot count in 16s and there-

fore have a hard time finding valid subnets, hosts, and broadcast 

addresses with the Class C 255.255.255.240 mask. You’d be wise to 

study this mask.