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In this, the IP address is followed by a slash and the number (in decimal) of bits used for the network part, also called the routing prefix. For example, an IPv4 address and its subnet mask may be 192.0.2.1 and 255.255.255.0 , respectively.
In the original design of IPv4, an IP address was divided into two parts: the network identifier was the most significant octet of the address, and the host identifier was the rest of the address. The latter was also called the rest field. This structure permitted a maximum of 256 network identifiers, which was quickly found to be inadequate.
The IP address of the destination is used to make decisions about routing IP packets to other networks. IPv6 is the successor to the first addressing infrastructure of the Internet, Internet Protocol version 4 (IPv4). In contrast to IPv4, which defined an IP address as a 32-bit value, IPv6 addresses have a size of 128 bits.
Such a dual-stack DNS server holds IPv4 addresses in the A records and IPv6 addresses in the AAAA records. Depending on the destination that is to be resolved, a DNS name server may return an IPv4 or IPv6 IP address, or both. A default address selection mechanism, or preferred protocol, needs to be configured either on hosts or the DNS server.
This remains the dominant internetworking protocol in use in the Internet Layer; the number 4 identifies the protocol version, carried in every IP datagram. IPv4 is defined in RFC 791 (1981). Version number 5 was used by the Internet Stream Protocol, an experimental streaming protocol that was not adopted. [7] The successor to IPv4 is IPv6.
The number of subnets available and the number of possible hosts in a network may be readily calculated. For instance, the 192.168.5.0 / 24 network may be subdivided into the following four / 26 subnets. The highlighted two address bits become part of the network number in this process.
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IP addresses in dot-decimal notation are also presented in CIDR notation, in which the IP address is suffixed with a slash and a number, used to specify the length of the associated routing prefix. For example, 127.0.0.1/8 specifies that the IP address has an eight-bit routing prefix, and therefore the subnet mask 255.0.0.0.