Historically, IP addresses have been assigned in the form of network numbers of class A, B, or C. With the introduction of CIDR (Classless Inter-Domain Routing) classful restrictions are no longer valid. Address space is now allocated and assigned on bit boundaries. The following table illustrates this:
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+----------------------------------------------+
|addrs bits pref mask |
+----------------------------------------------+
| 1 0 /32 255.255.255.255 |
| 2 1 /31 255.255.255.254 |
| 4 2 /30 255.255.255.252 |
| 8 3 /29 255.255.255.248 |
| 16 4 /28 255.255.255.240 |
| 32 5 /27 255.255.255.224 |
| 64 6 /26 255.255.255.192 |
| 128 7 /25 255.255.255.128 |
| 256 8 /24 255.255.255 |1
| 512 9 /23 255.255.254 |2
| 1K 10 /22 255.255.252 |4
| 2K 11 /21 255.255.248 |8
| 4K 12 /20 255.255.240 |16
| 8K 13 /19 255.255.224 |32
| 16K 14 /18 255.255.192 |64
| 32K 15 /17 255.255.128 |128
| 64K 16 /16 255.255 |256
| 128K 17 /15 255.254 |
| 256K 18 /14 255.252 |
| 512K 19 /13 255.248 |
| 1M 20 /12 255.240 |
| 2M 21 /11 255.224 |
| 4M 22 /10 255.192 |
| 8M 23 /9 255.128 |
| 16M 24 /8 255 |
| 32M 25 /7 254 |
| 64M 26 /6 252 |
| 128M 27 /5 248 |
| 256M 28 /4 240 |
| 512M 29 /3 224 |
|1024M 30 /2 192 |
+----------------------------------------------+
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