Frequent Questions      

CCNA™: Subnet masking

 

Download practice tests: CCNA™ certification exams

 

Subnetting an IP Network is done primarily for better utilization of available IP address space, and routing purpose. Other reasons include better organization, use of different physical media (such as Ethernet, WAN, etc.),  and securing network resources.

A subnet mask enables you to identify the network and node parts of the address. The network bits are represented by the 1s in the mask, and the node bits are represented by the 0s. A logical AND operation between the IP address and the subnet mask provides the Network Address.

For example, using our test IP address and the default Class C subnet mask, we get:

192.189.210.078: 1100 0000.1011 1101.1101 0010.0100 1110 Class C IP Address

255.255.255.000: 1111 1111.1111 1111.1111 1111.0000 0000 Default Class C subnet mask

192.189.210.0         1100 0000 1011 1101 1101 0010 0000 0000

As can be seen above, by using and AND operator, we can compute the network portion of an IP address.  The network portion for the IP address given in the above example is 192.189.210.0, and the host portion of the IP address is 078.

Given below is a table that provides binary equivalent of decimal values. 

For binary conversion, take first octet of a given IP address (in dotted decimal form), and lookup the binary value. Then take the second octet and lookup the binary value, and so on.

Binary Conversion Table

Decimal

Binary

Decimal

Binary

Decimal

Binary

Decimal

Binary

0

0000 0000

64

0100 0000

128

1000 0000

192

1100 0000

1

0000 0001

65

0100 0001

129

1000 0001

193

1100 0001

2

0000 0010

66

0100 0010

130

1000 0010

194

1100 0010

3

0000 0011

67

0100 0011

131

1000 0011

195

1100 0011

4

0000 0100

68

0100 0100

132

1000 0100

196

1100 0100

5

0000 0101

69

0100 0101

133

1000 0101

197

1100 0101

6

0000 0110

70

0100 0110

134

1000 0110

198

1100 0110

7

0000 0111

71

0100 0111

135

1000 0111

199

1100 0111

8

0000 1000

72

0100 1000

136

1000 1000

200

1100 1000

9

0000 1001

73

0100 1001

137

1000 1001

201

1100 1001

10

0000 1010

74

0100 1010

138

1000 1010

202

1100 1010

11

0000 1011

75

0100 1011

139

1000 1011

203

1100 1011

12

0000 1100

76

0100 1100

140

1000 1100

204

1100 1100

13

0000 1101

77

0100 1101

141

1000 1101

205

1100 1101

14

0000 1110

78

0100 1110

142

1000 1110

206

1100 1110

15

0000 1111

79

0100 1111

143

1000 1111

207

1100 1111

 

 

 

 

 

 

 

 

16

0001 0000

80

0101 0000

144

1001 0000

208

1101 0000

17

0001 0001

81

0101 0001

145

1001 0001

209

1101 0001

18

0001 0010

82

0101 0010

146

1001 0010

210

1101 0010

19

0001 0011

83

0101 0011

147

1001 0011

211

1101 0011

20

0001 0100

84

0101 0100

148

1001 0100

212

1101 0100

21

0001 0101

85

0101 0101

149

1001 0101

213

1101 0101

22

0001 0110

86

0101 0110

150

1001 0110

214

1101 0110

23

0001 0111

87

0101 0111

151

1001 0111

215

1101 0111

24

0001 1000

88

0101 1000

152

1001 1000

216

1101 1000

25

0001 1001

89

0101 1001

153

1001 1001

217

1101 1001

26

0001 1010

90

0101 1010

154

1001 1010

218

1101 1010

27

0001 1011

91

0101 1011

155

1001 1011

219

1101 1011

28

0001 1100

92

0101 1100

156

1001 1100

220

1101 1100

29

0001 1101

93

0101 1101

157

1001 1101

221

1101 1101

30

0001 1110

94

0101 1110

158

1001 1110

222

1101 1110

31

0001 1111

95

0101 1111

159

1001 1111

223

1101 1111

 

 

 

 

 

 

 

 

32

0010 0000

96

0110 0000

160

1010 0000

224

1110 0000

33

0010 0001

97

0110 0001

161

1010 0001

225

1110 0001

34

0010 0010

98

0110 0010

162

1010 0010

226

1110 0010

35

0010 0011

99

0110 0011

163

1010 0011

227

1110 0011

36

0010 0100

100

0110 0100

164

1010 0100

228

1110 0100

37

0010 0101

101

0110 0101

165

1010 0101

229

1110 0101

38

0010 0110

102

0110 0110

166

1010 0110

230

1110 0110

39

0010 0111

103

0110 0111

167

1010 0111

231

1110 0111

40

0010 1000

104

0110 1000

168

1010 1000

232

1110 1000

41

0010 1001

105

0110 1001

169

1010 1001

233

1110 1001

42

0010 1010

106

0110 1010

170

1010 1010

234

1110 1010

43

0010 1011

107

0110 1011

171

1010 1011

235

1110 1011

44

0010 1100

108

0110 1100

172

1010 1100

236

1110 1100

45

0010 1101

109

0010 1101

173

1010 1101

237

1010 1101

46

0010 1110

110

0110 1110

174

1010 1110

238

1110 1110

47

0010 1111

111

0110 1111

175

1010 1111

239

1110 1111

 

 

 

 

 

 

 

 

48

0011 0000

112

0111 0000

176

1011 0000

240

1111 0000

49

0011 0001

113

0111 0001

177

1011 0001

241

1111 0001

50

0011 0010

114

0111 0010

178

1011 0010

242

1111 0010

51

0011 0011

115

0111 0011

179

1011 0011

243

1111 0011

52

0011 0100

116

0111 0100

180

1011 0100

244

1111 0100

53

0011 0101

117

0111 0101

181

1011 0101

245

1111 0101

54

0011 0110

118

0111 0110

182

1011 0110

246

1111 0110

55

0011 0111

119

0111 0111

183

1011 0111

247

1111 0111

56

0011 1000

120

0111 1000

184

1011 1000

248

1111 1000

57

0011 1001

121

0111 1001

185

1011 1001

249

1111 1001

58

0011 1010

122

0111 1010

186

1011 1010

250

1111 1010

59

0011 1011

123

0111 1011

187

1011 1011

251

1111 1011

60

0011 1100

124

0111 1100

188

1011 1100

252

1111 1100

61

0011 1101

125

0111 1101

189

1011 1101

253

1111 1101

62

0011 1110

126

0111 1110

190

1011 1110

254

1111 1110

63

0011 1111

127

0111 1111

191

1011 1111

255

1111 1111

Example Question: Which of the following is a Class C IP address?

      A. 10.10.14.118

      B. 135.23.112.57

      C. 191.200.199.199

      D. 204.67.118.54

 

                          

Correct Answer: D.

Explanation:

IP addresses are written using decimal numbers separated by decimal points. This is called dotted decimal notation of expressing IP addresses. The different classes of IP addresses is as below:

Class

Format 

Leading Bit  pattern

Network address Range   

Maximum  networks

Maximum hosts     

A

N.H.H.H

 0

 0-126

 127

 16,777,214

B

N.N.H.H   

 10

 128-191

 16,384

 65,534

C

N.N.N.H

 110

 192-223

 2,097,152

 254

Network address of all zeros means "This network or segment".

Network address of all 1s means " all networks", same as hexadecimal of all Fs.

Network number 127 is reserved for loopback tests.

Host (Node) address of all zeros mean "This Host (Node)".

Host (Node) address of all 1s mean "all Hosts (Nodes) " on the specified network.

 

 


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