Simple class C cheat sheet
(although it will also work for all class with a little addition)
192 224
240 248
252 254
255 Subnet Mask
128
64
32
16 8
4
2
1 Number of Bits in the Subnet
Mask
126
62
30
14
6
2
N/A N/A Hosts per Subnet
2
6
14
30
62 126
254 Subnets
The above table
is easy to cheat by doing the following. Practice makes perfect. Simple write
down the right side of a piece of paper the 4 components; Mask, Bits, Host,
Subs. Then fill in the bits first, RIGHT to LEFT, not left to right, double
each bit as you go. Then starting at 128, add 64 and put the result above the
64. Then add that result to the next bits of 32 and put that result above the
32, and continue to the end. Then starting at 128 bits, subtract 2 (2^n-2
formula to calculate hosts-see below) and put that number below the 128.
Subtract 2 from each number in the bits row and put it underneath that number
in the bits row. Obviously 2 and 1 result in 0 or less. The subnet row is found
by dropping down at 14 and reversing the entire host row around it.
This is a quick
and dirty class c table for quick reference. If you have a class c address with
a mask of 255.255.255.224, you will have 32 bits in the mask, 30 hosts per
subnet and 6 subnets in the network.
It will work for
any class of address or number of octets the network number uses. The subnets
are always the same. The number of hosts is what changes. You can use the table
above o quickly get the subnet and then based on the number of bits in the host
portion, you can add up the number hosts. For example, a class B of 172.16.0.0
network with mask of 255.255.248.0 will have 30 subnets. The hosts in the last
octet is all of them which is 256. No we don't subtract 2 yet (2^n-2). There
are 3 more bit positions, 1, 2 and 4 in the host portion in the next octet
(where the 248 is). so multipy it by 2, 3 more times, 256*2=512, 512*2=1024,
1024*2=2048. Now subtract 2. 2048-2= 2046. You have 2046 hosts to use in 30
subnets.
Decimal Subnets A
hosts NB B hosts NB C hosts
.128
1
8,388,606
/9
32,766
/17
128
/25
.192 2 4,194,302 /1016,382 /18 62
.224 6 2,097,150 /118,190 /1930
.240 14 1,048,574 /124,094
/20 14
.248 30 524,286 /132,046
/21 6
.252 62 262,142 /141,022
/22 2
.254 126 131,070 /15510 /23NA
.255 254 65,534 /16254 /24NA
128 10000000 A 1-126
64 01000000 Loopback
127
32 00100000 B 128-191
16 00010000 C 192-223
8 00001000 D 224-239
4 00000100 E 240-247
2 00000010
1 00000001
The
following attachments are additional TCP/IP study aids. Right click the icon
and make your selection.
IP-Summary.pdf |
TCPIP-Sub.pdf |
NOTE: The 2^n-2 formula
comes from the fact that the first node of a network number indicates the
network and is unusable and the last node is the broadcast address for the
network. For network 192.168.1.0, 192.168.1.0 indicates the network.
192.168.1.255 is the bradcast address.