Understand IP addresses and CIDR notation

Today we will discuss about IP addresses and CIDR (Classless Inter-Domain Routing) notation.

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The first address of a subnet block (all 0’s) is called the network address or network ID. The last address (all 1’s) is the broadcast address of the network. Typically the network address +1 or the broadcast address -1 is the gateway to the internet. This leaves us with total number of IP numbers -3 left over for host address within a sub net block. That’s why you either get 1 IP (4-3= 1) or if you ask for one more you get 5 (8-3=5).

For example, 192.168.1.0/25 would include all address between 192.168.1.0 and 192.168.1.12.

Following is the mask table for CIDR notation:

Mask = /24

0-255

Mask = /25

0-127
128-255

Mask = /26

0-63
64-127
128-191
192-255

Mask = /27

0-31
32-63
64-95
96-127
128-159
160-191
192-223
224-255

Mask = /28

0-15
16-31
32-47
48-63
64-79
80-111
112-127
128-143
144-159
160-175
176-191
192-207
208-223
224-239
240-255

Mask = /29

0-7
8-15
16-23
24-31
32-39
40-47
48-55
56-63
64-71
72-79
80-87
88-95
96-103
104-111
112-119
120-127
128-135
136-143
144-151
152-159
160-167
168-175
176-183
184-191
192-199
200-207
208-215
216-223
224-231
232-239
240-247
248-255

Mask = /30

0-3
4-7
8-11
12-15
16-19
20-23
24-27
28-31
32-35
36-39
40-43
44-47
48-51
52-55
56-59
60-63
64-67
68-71
72-75
76-79
80-83
84-87
88-91
92-95
96-99
100-103
104-107
108-111
112-115
116-119
120-123
124-127
128-131
132-135
136-139
140-143
144-147
148-151
152-155
156-159
160-163
164-167
168-171
172-175
176-179
180-183
184-187
188-191
192-195
196-199
200-203
204-207
208-211
212-215
216-219
220-223
224-227
228-231
232-235
236-239
240-243
244-247
248-251
252-255

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