What does "192.168.1.1/21" mean?

Question

When they write a slash followed by a number after an IP address, what does that mean?

Accepted Answer

The part after the slash is how many subnet mask bits to use. Since the use of classless routing you use slash instead of saying class A or B whatever. Example:

```
192.168.1.1/24 is 192.168.1.1 255.255.255.0
```

`255.255.255.0` is using `24` of the `32 bits` to create the subnet.

in binary it looks like this:

```
11111111.11111111.11111111.00000000
```

so a `/30` would look like:

```
255.255.255.252 or in binary 
11111111.11111111.11111111.11111100
```

the remaing 00 is for hosts; the 1's are the network.

What does "192.168.1.1/21" mean?

Solution

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