The number 255 appears in IP addresses primarily because it represents the maximum value that can be stored in one byte (8 bits). Specifically, in the context of IP addresses, 255 is used as a broadcast address.
Here's a breakdown:
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IP Address Structure: An IPv4 address is a 32-bit address, typically written in dotted decimal notation as four octets (bytes) separated by periods (e.g., 192.168.1.255).
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Byte Representation: Each octet represents a number from 0 to 255. This range comes from the fact that each octet consists of 8 bits. With 8 bits, you can represent 28 = 256 different values. Since we start counting from 0, the maximum value is 255.
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Broadcast Address: The IP address with all host bits set to 1 (which translates to 255 in decimal) is used to broadcast a message to every host on a network. According to the reference, "the 255 address (in binary notation, a host address of all ones) is used to broadcast a message to every host on a network."
Broadcast Addresses Explained
A broadcast address allows a device to send a single message that is received by every device on the network. This is useful for certain network functions, such as:
- Network Discovery: A device can send a broadcast message to discover other devices or services on the network.
- Routing Updates: Routers may use broadcast messages to share routing information with other routers on the network.
Why Not Use the First and Last Address for Hosts?
As highlighted in the provided information, the first and last address in any network or subnet can't be assigned to any individual host.
- Network Address: The first address (usually ending in .0) identifies the network itself.
- Broadcast Address: The last address (usually ending in .255) is the broadcast address.
Therefore, if you have a network, you can assign IP addresses to the hosts in the network excluding the first and the last address. The reference highlights that you will "be able to give IP addresses to 254 hosts".
Example
Consider a Class C network with a default subnet mask of 255.255.255.0. Let's say the network address is 192.168.1.0.
| IP Address | Purpose |
|---|---|
| 192.168.1.0 | Network Address |
| 192.168.1.1 | Host 1 |
| ... | ... |
| 192.168.1.254 | Host 254 |
| 192.168.1.255 | Broadcast Address |
In this example, IP addresses from 192.168.1.1 to 192.168.1.254 can be assigned to hosts. 192.168.1.0 represents the network itself, and 192.168.1.255 is the broadcast address for that network.
