There is one last detail:
there is a standard notation for groups of IP addresses, sometimes
called a `network address'. Just like a phone number can be broken up
into an area prefix and the rest, we can divide an IP address into a
network prefix and the rest.
It used to be that people would talk about `the 1.2.3 network', meaning all 256 addresses from 1.2.3.0 to 1.2.3.255. Or if that wasn't a big enough network, they might talk about the `1.2 network' which meant all addresses from 1.2.0.0 to 1.2.255.255.
We usually don't write `1.2.0.0 - 1.2.255.255'. Instead, we shorten it to `1.2.0.0/16'. This weird `/16' notation (it's called a `netmask') requires a little explanation.
Each number between the dots in an IP address is actually 8 binary digits (00000000 to 11111111): we write them in decimal form to make it more readable for humans. The `/16' means that the first 16 binary digits is the network address, in other words, the `1.2.' part is the the network (remember: each digit represents 8 binary digits). This means any IP address beginning with `1.2.' is part of the network: `1.2.3.4' and `1.2.3.50' are, and `1.3.1.1' is not.
To make life easier, we usually use networks ending in `/8', `/16' and `/24'. For example, `10.0.0.0/8' is a big network containing any address from 10.0.0.0 to 10.255.255.255 (over 16 million addresses!). 10.0.0.0/16 is smaller, containing only IP addresses from 10.0.0.0 to 10.0.255.255. 10.0.0.0/24 is smaller still, containing addresses 10.0.0.0 to 10.0.0.255.
To make things confusing, there is another way of writing netmasks. We can write them like IP addresses:
Finally, it's worth noting that the very highest IP address in any network is reserved as the `broadcast address', which can be used to send a message to everyone on the network at once.
Here is a table of network masks:
It used to be that people would talk about `the 1.2.3 network', meaning all 256 addresses from 1.2.3.0 to 1.2.3.255. Or if that wasn't a big enough network, they might talk about the `1.2 network' which meant all addresses from 1.2.0.0 to 1.2.255.255.
We usually don't write `1.2.0.0 - 1.2.255.255'. Instead, we shorten it to `1.2.0.0/16'. This weird `/16' notation (it's called a `netmask') requires a little explanation.
Each number between the dots in an IP address is actually 8 binary digits (00000000 to 11111111): we write them in decimal form to make it more readable for humans. The `/16' means that the first 16 binary digits is the network address, in other words, the `1.2.' part is the the network (remember: each digit represents 8 binary digits). This means any IP address beginning with `1.2.' is part of the network: `1.2.3.4' and `1.2.3.50' are, and `1.3.1.1' is not.
To make life easier, we usually use networks ending in `/8', `/16' and `/24'. For example, `10.0.0.0/8' is a big network containing any address from 10.0.0.0 to 10.255.255.255 (over 16 million addresses!). 10.0.0.0/16 is smaller, containing only IP addresses from 10.0.0.0 to 10.0.255.255. 10.0.0.0/24 is smaller still, containing addresses 10.0.0.0 to 10.0.0.255.
To make things confusing, there is another way of writing netmasks. We can write them like IP addresses:
Finally, it's worth noting that the very highest IP address in any network is reserved as the `broadcast address', which can be used to send a message to everyone on the network at once.
Here is a table of network masks:
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