IP Address Blocks

There are 4 classes of IPv4 networks:

A Class A network can have 16,777,214 computers on it. It has an IP address that starts with a number in the range 0 to 126. 127 is reserved for the local loopback. (Only 127.0.0.1 is ever used although this may change in the future).

A Class B network can have 65,534 computers on it. It has an IP address that starts in the range of 128 to 191.

A Class C network can have 254 computers on it. This network is often used for many home networks and most company networks. It has an IP address that starts with a number from 192 to 223.

Class D (224–239) is used for multicast and Class E (240–254) is reserved.

Within the above classes are sets of private network addresses:

• Any address that starts with 10
• Any address that starts 172 and where the second number is 16–31.
• Any address that starts with 192.168

Routers will generally discard any traffic to private network addresses. So,in order to talk to other devices, your router does something called Network Address Translation or NATtin. You can read more about Network Address Translation and how it works here.

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