Slash notation for subnet masks quick reference
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One thing I can never remember quickly is slash notation, it is not difficult but I just can’t seem to remember it quickly! So here is a quick reference guide. In slash notation, a single number indicates how many bits of the IP address identify the network the host is on. A netmask of 255.255.255.0 has a netmask of 8 + 8 + 8 = 24.
For example, writing 192.168.42.23/24 is the same as specifying an IP address of 192.168.42.23 with a corresponding netmask of 255.255.255.0. Often you have to enter the netmask as slash notation, an easy task with the usual 255.255.255.0. However if your network doesn’t have 255 hosts, for example only 8 hosts, then the netmask will be 255.255.255.248.
The following table lists the variable length subnets from 1 to 32, the CIDR [3] representation form (/xx) and the Decmial equivalents. (M = Million, K=Thousand, A,B,C= traditional class values)
| Hex | CIDR | Decimal | # Of Addresses | Classfull |
| 80.00.00.00 | /1 | 128.0.0.0 |
2048 M
|
128 A
|
| C0.00.00.00 | /2 | 192.0.0.0 |
1024 M
|
64 A
|
| E0.00.00.00 | /3 | 224.0.0.0 |
512 M
|
32 A
|
| F0.00.00.00 | /4 | 240.0.0.0 |
256 M
|
16 A
|
| F8.00.00.00 | /5 | 248.0.0.0 |
128 M
|
8 A
|
| FC.00.00.00 | /6 | 252.0.0.0 |
64 M
|
4 A
|
| FE.00.00.00 | /7 | 254.0.0.0 |
32 M
|
2 A
|
| FF.00.00.00 | /8 | 255.0.0.0 |
16 M
|
1 A
|
| FF.80.00.00 | /9 | 255.128.0.0 |
8 M
|
128 B
|
| FF.C0.00.00 | /10 | 255.192.0.0 |
4 M
|
64 B
|
| FF.E0.00.00 | /11 | 255.224.0.0 |
2 M
|
32 B
|
| FF.F0.00.00 | /12 | 255.240.0.0 |
1024 K
|
16 B
|
| FF.F8.00.00 | /13 | 255.248.0.0 |
512 K
|
8 B
|
| FF.FC.00.00 | /14 | 255.252.0.0 |
256 K
|
4 B
|
| FF.FE.00.00 | /15 | 255.254.0.0 |
128 K
|
2 B
|
| FF.FF.00.00 | /16 | 255.255.0.0 |
64 K
|
1 B
|
| FF.FF.80.00 | /17 | 255.255.128.0 |
32 K
|
128 C
|
| FF.FF.C0.00 | /18 | 255.255.192.0 |
16 K
|
64 C
|
| FF.FF.E0.00 | /19 | 255.255.224.0 |
8 K
|
32 C
|
| FF.FF.F0.00 | /20 | 255.255.240.0 |
4 K
|
16 C
|
| FF.FF.F8.00 | /21 | 255.255.248.0 |
2 K
|
8 C
|
| FF.FF.FC.00 | /22 | 255.255.252.0 |
1 K
|
4 C
|
| FF.FF.FE.00 | /23 | 255.255.254.0 |
512
|
2 C
|
| FF.FF.FF.00 | /24 | 255.255.255.0 |
256
|
1 C
|
| FF.FF.FF.80 | /25 | 255.255.255.128 |
128
|
1/2 C
|
| FF.FF.FF.C0 | /26 | 255.255.255.192 |
64
|
1/4 C
|
| FF.FF.FF.E0 | /27 | 255.255.255.224 |
32
|
1/8 C
|
| FF.FF.FF.F0 | /28 | 255.255.255.240 |
16
|
1/16 C
|
| FF.FF.FF.F8 | /29 | 255.255.255.248 |
8
|
1/32 C
|
| FF.FF.FF.FC | /30 | 255.255.255.252 |
4
|
1/64 C
|
| FF.FF.FF.FE | /31 | 255.255.255.254 |
2
|
1/128 C
|
| FF.FF.FF.FF | /32 | 255.255.255.255 |
This is a single host route
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