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SourceCode for DbaseProgrammers.com

Figuring out a subnet

Subnets divide blocks of IP addresses into separate networks. IP addresses are set in blocks
of 256 starting with 0, and ending with 255. With IPv4, the IP address may look like this.
192.168.1.55. The math in finding a subnet is 256 minus the amount of IP addresses you need.
IP addresses need to group in sets of  1, 2, 4, 8, 16, 32, 64, 128, 256...

For example, if you want to use all the IP addresses in a class C (256 Ips) then the equation will be
256
-256
255.255.255.     0
If you wanted to use blocks of 32 IP addresses then
256
-  32
255.255.255. 224
If you wanted to use blocks of 32 IP addresses then be
256
-    8
255.255.255. 248
This math even works for class A, B, and C networks. If you wanted to use 2 class C networks that are side by side
256  256
-    2 -256
255.255.254.     0

The first IP address in a block of IP addresses is called the "network".
The last number in an IP block is called the "broadcast". Niether of them are usable IP addresses.

256
0                                              All IP Addresses in a Class C                                     255
0                                                            127 128                                                       255
0                           63 64                       127 128                   191 192                     255
0         31 32       63 64         95 96     127 128   159 160  191 192   223 224    255
0-15,  16-31, 32-47, 48-63, 64-79, 80-95, 96-111, 112-127, 128-143,144-159, 160-175, 176-191, 192-207, 208-223, 224-234, 240-255
128
64
32
16
8
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, 232-234, 240-247, 248-255

     Subnets have to be in blocks that are divisible by the IP address blocks. Say we want 32
IP addresses. 0-31, 32-63 are blocks that are ok for a subnet of 255.255.255.224.

16-47 is not... Your network will be split into two separate blocks. 16-31, 32-47 with both
31 and 32 unusable. 31 being the "broadcast" of the first network, and 32 the "network" of the
second. There are devices though that bridges the gap between networks. They are called routers.
Routers need to use NAT (Network Address Translation) to create the bridge.

Author: Jeremy Martin

 



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