IP addresses in IPv4 are 32 bits long, and are often written in dotted decimal notation, where each byte is represented by a decimal number between 0 and 255, and these values are separated by a period (a full stop), for example 192.168.11.4 The address for a specific host must have a unique 32-bit address and this in composed from a network prefix and a host part. All of the hosts within a network share a common network prefix, and each host within a network must have a unique host part of the overall address. The boundary between the network part and the host part of the address can occur at different places within the overall 32-bit address, and where this boundary lies for a specific host is defined by the mask for the address. Traditionally these addresses have been defined by network prefix ending on each of the byte boundaries within the addressing field:
- Class A addresses use the most significant byte for the network part, the three remaining byte for the host part of the address
- Class B addresses use the most significant two bytes for the network part, and the remaining bytes for the host part of the address
- Class C addresses use the most significant three bytes for the network part, and the remaining byte for the host part of the address
- Class D addresses are special addresses reserved for multicast; many hosts can listen on the same multicast address for packets sent from one sender
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