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Content:: Network Subnets

Network Subnet

Traditional "Classful" Networks were grouped into three primary classes. A class "A", class "B", class "C". Each class divided an IP address by the bit count. An IP Address Consists of 4 block of numbers ranging from 1 - 255 with each block seperated by a period. Each class controls a specific block of IP Addresses like (A.B.C.#). A class C network would consist of 255 addresses an example would be 10.0.0.1 to 10.0.0.255. A class B network would be 10.0.0.1 to 10.0.255.255 A class A network would be 10.0.0.0 to 10.255.255.255.

In the 1990's subnets were created to split the larger networks apart. A class C networks subnet would be 255.255.255.0. It is possible to split a class C address (containing 256 IP addresses) in half with a subnet of 255.255.255.128 containing 128 IP addresses. Each subnet will by default use two IP addresses one for the router and one for the broadcast.

Table of Contents CIDR

Each block in a subnet can be a number from 0 to 255 a total of 256 possibilities. A "Classless Inter-Domain Routing" Notation commonly known as CIDR is a shorthand notation for the subnet. A CIDR Notation can be calculated by counting the bits in each block of the subnet.

A byte consists of 8 bits. Each bit is basically an On and off switch with each byte containing 8 On and Off switches. The possible combinations of 8 On and Off switches is 256. A binary Number appears as a sequence of 8 characters, each character can be a 0 or a 1. A binary number commonly appears as 0000-0000.

Binary Number :0000-0000
Bit Count Per Placement :128643216-8421

Using the above table we can translate the numbers 0-255 to Binary or vice versa. In Binary the number 0 will appear as 0000-0000, 3 will appears as 0000-0011, 128 will appear as 1000-0000, 255 as 1111-1111.

Now that we know how to count bits from a subnet number sequence we can look at a class C network which has the subnet of 255.255.255.0. Each 255 will appear as 1111-1111 and the 0 appears as 0000-0000 in binary. Each one is considered as a bit we can count for the CIDR. Lets look at the subnet as a binary byte.
1111-1111.1111-1111.1111-1111.0000-0000 If we count every number 1 we can see a total of 24. Instead of counting each bit we can easily figure out each block contains 8 ones. With three blocks set to 255 we can multiple 8 by 3 to get the same answer.

Subnet Table

Subnet Mask IP Addresses per Network Number of Hosts CIDR Notation
255.0.0.016,777,21616,777,214/8
255.128.0.08,388,6088,388,606/9
255.192.0.04,194,3044,194,302/10
255.224.0.02,097,1522,097,150/11
255.240.0.01,048,5761,048,574/12
255.248.0.0524,288524,286/13
255.252.0.0262,144262,142/14
255.254.0.0131,072131,070/15
255.255.0.065,53665,534/16
255.255.128.032,76632,764/17
255.255.192.016,38416,382/18
255.255.224.08,1928,190/19
255.255.240.04,0964,094/20
255.255.248.02,0482,046/21
255.255.252.01,0241,022/22
255.255.254.0512510/23
255.255.255.0256254/24
255.255.255.128128126/25
255.255.255.1926462/26
255.255.255.2243230/27
255.255.255.2401614/28
255.255.255.24886/29
255.255.255.25242/30
255.255.255.25422*/31
255.255.255.25511*/32
Class C Subnet break down

/25 - 2 subnets
Subnet ID/Network NumberIP RangeBroadcast
01-126127
128129-254255

/26 - 4 subnets
Subnet ID/Network NumberIP RangeBroadcast
01-6263
6465-126127
128129-190191
192193-254255

/27 - 8 subnets
Subnet ID/Network NumberIP RangeBroadcast
01-3031
3233-6263
6465-9495
9697-126127
128129-158159
160161-190191
192193-222223
224225-254255

/28 - 16 subnets
Subnet ID/Network NumberIP RangeBroadcast
01-1415
1617-3031
3233-4647
4849-6263
6465-7879
8081-9495
9697-110111
112113-126127
128129-142143
144145-158159
160161-174175
176177-190191
192193-206207
208209-222223
224225-238239
240241-254255

/29 - 32 subnets
Subnet ID/Network NumberIP RangeBroadcast
01-67
89-1415
1617-2223
2425-3031
3233-3839
4041-4647
4849-5455
5657-6263
6465-7071
7273-7879
8081-8687
8889-9495
9697-102103
104105-110111
112113-118119
120121-126127
128129-134135
136137-142143
144145-150151
152153-158159
160161-166167
168169-174175
176177-182183
184185-190191
192193-198199
200201-206207
208209-214215
216217-222223
224225-230231
232233-238239
240241-246247
248249-254255

/30 - 64 subnets
Subnet ID/Network NumberIP RangeBroadcast
01-23
45-67
89-1011
1213-1415
1617-1819
2021-2223
2425-2627
2829-3031
3233-3435
3637-3839
4041-4243
4445-4647
4849-5051
5253-5455
5657-5859
6061-6263
6465-6667
6869-7071
7273-7475
7677-7879
8081-8283
8485-8687
8889-9091
9293-9495
9697-9899
100101-102103
104105-106107
108109-110111
112113-114115
116117-118119
120121-122123
124125-126127
128129-130131
132133-134135
136137-138139
140141-142143
144145-146147
148149-150151
152153-154155
156157-158159
160161-162163
164165-166167
168169-170171
172173-174175
176177-178179
180181-182183
184185-186187
188189-190191
192193-194195
196197-198199
200201-202203
204205-206207
208209-210211
212213-214215
216217-218219
220221-222223
224225-226227
228229-230231
232233-234235
236237-238239
240241-242243
244245-246247
248249-250251
252253-254255
References

Last Modified: 2012-10-21