How is a subnet mask created?

Answer 1

In computer networks based on the internet Protocol Suite, a subnetwork, or subnet, is a portion of the network's computers and network devices that have a common, designated IP address routing prefix (cf. Classless Inter-Domain Routing, CIDR). A routing prefix is the sequence of leading bits of an IP address that precede the portion of the address used as host identifier (or rest field in early Internet terminology). In IPv4 installations, the routing prefix is often expressed as a "subnet mask", which is a bit mask covering the number of bits used in the prefix. It is frequently expressed in quad-dotted decimal representation, e.g., 255.255.255.0 is the subnet mask for the 192.168.1.0 network with a 24-bit routing prefix (192.168.1.0/24). Subnet masks in IPv4 do not have to have consecutive bits set, e.g., a subnet mask of "11111111001100110000000000000000" (binary) is permissible, albeit of little additional value, as it does not provide for more efficient address space utilization and cannot be represented in CIDR notation. IPv6 does not use subnet masks and such non-standard routing prefixes are not possible. The routing prefix of a subnet is often further divided into the network's network identifier and a subnet identifier. The network identifier is the leading set of address bits that is common to the prefixes of all subnets in the network. This would typically be the CIDR routing prefix of an organization's entire address space allocation. The subnet identifier consists of the remaining bits in a subnet's prefix after the network identifier. In the cited example, 192.168.0.0 is the network identifier (and 255.255.0.0 the network mask) and "1" is the subnet identifier. In a large organization's network, without subnetting, traffic levels can grow enough that excessive rates of Ethernet packet collisions become a bottle neck due to the nature of Ethernet (cf. carrier sense multiple access with collision detection). For this reason, subnetting can be used to break the network into smaller more efficient subnets. Such subnets can be arranged hierarchically, with the organization's network address space (see also Autonomous System) partitioned into a tree-like structure. Routers are used to manage traffic and constitute borders between subnets. Communication is of specific link-local character (Ethernet broadcast) only within the smallest subnet. A typical subnet is a physical network served by one router, for instance an Ethernet network (consisting of one or several Ethernet segments or local area networks, interconnected by network switches and network bridges) or a Virtual Local Area Network (VLAN). However, subnetting allows the network to be logically divided regardless of the physical layout of a network, since it is possible to divide a physical network into several subnets by configuring different host computers to use different routers. While improving network performance, subnetting increases routing complexity, since each locally connected subnet is typically represented by one row in the routing tables in each connected router. However, with intelligent design of the network, routes to collections of more distant subnets within the branches of a tree-hierarchy can be aggregated by single routes. Subnetting was originally introduced before the introduction of classful network addresses in IPv4 to allow a single larger network to have a number of smaller networks within it, controlled by several routers. Existing subnetting functionality in routers made the introduction of Classless Inter-Domain Routing seamless.

Answer 2

The subnet mask is created by placing a binary 1 in each bit position that represents the network portion and placing a binary 0 in each bit position that represents the host portion.