File Name: ip addressing and subnetting .zip
Internetworking Protocol IP addresses are the unique numeric identifiers required of every device connected to the Internet. They allow for the precise routing of data across very complex worldwide internetworks. IP address allocation within an organization requires a lot of long-term planning. This timely publication addresses the administrator and engineer's need to know how IP 6 impacts their enterprise networks.
A subnetwork or subnet is a logical subdivision of an IP network. Computers that belong to a subnet are addressed with an identical most-significant bit -group in their IP addresses. This results in the logical division of an IP address into two fields: the network number or routing prefix and the rest field or host identifier. The rest field is an identifier for a specific host or network interface. For example, Addresses in the range For IPv4, a network may also be characterized by its subnet mask or netmask , which is the bitmask that when applied by a bitwise AND operation to any IP address in the network, yields the routing prefix.
Subnet masks are also expressed in dot-decimal notation like an address. Traffic is exchanged between subnetworks through routers when the routing prefixes of the source address and the destination address differ. A router serves as a logical or physical boundary between the subnets. The benefits of subnetting an existing network vary with each deployment scenario. In the address allocation architecture of the Internet using CIDR and in large organizations, it is necessary to allocate address space efficiently.
Subnetting may also enhance routing efficiency, or have advantages in network management when subnetworks are administratively controlled by different entities in a larger organization. Subnets may be arranged logically in a hierarchical architecture, partitioning an organization's network address space into a tree-like routing structure, or other structures such as meshes.
Computers participating in a network such as the Internet each have at least one network address. Usually, this address is unique to each device and can either be configured automatically with the Dynamic Host Configuration Protocol DHCP by a network server, manually by an administrator, or automatically by stateless address autoconfiguration.
An address fulfills the functions of identifying the host and locating it on the network. The most common network addressing architecture is Internet Protocol version 4 IPv4 , but its successor, IPv6 , has been increasingly deployed since approximately An IPv4 address consists of 32 bits, for readability written in a form consisting of four decimal octets separated by dots, called dot-decimal notation. An IPv6 address consists of bits written in hexadecimal notation and groupings of 16 bits, called hextets , separated by colons.
An IP address is divided into two logical parts, the network prefix and the host identifier. All hosts on a subnetwork have the same network prefix. This prefix occupies the most-significant bits of the address. The number of bits allocated within a network to the prefix may vary between subnets, depending on the network architecture. The host identifier is a unique local identification and is either a host number on the local network or an interface identifier.
This addressing structure permits the selective routing of IP packets across multiple networks via special gateway computers, called routers , to a destination host if the network prefixes of origination and destination hosts differ, or sent directly to a target host on the local network if they are the same.
Routers constitute logical or physical borders between the subnets, and manage traffic between them. Each subnet is served by a designated default router but may consist internally of multiple physical Ethernet segments interconnected by network switches. The routing prefix of an address is identified by the subnet mask , written in the same form used for IP addresses. For example, the subnet mask for a routing prefix that is composed of the most-significant 24 bits of an IPv4 address is written as For example, the IPv4 network In classful networking in IPv4, before the introduction of CIDR, the network prefix could be directly obtained from the IP address, based on its highest order bit sequence.
This determined the class A, B, C of the address and therefore the subnet mask. Since the introduction of CIDR, however, the assignment of an IP address to a network interface requires two parameters, the address and a subnet mask. Given an IPv4 source address, its associated subnet mask, and the destination address, a router can determine whether the destination is on-link or off-link.
The subnet mask of the destination is not needed, and is generally not known to a router. Since each locally connected subnet must be represented by a separate entry in the routing tables of each connected router, subnetting increases routing complexity. However, by careful design of the network, routes to collections of more distant subnets within the branches of a tree hierarchy can be aggregated into a supernetwork and represented by single routes.
An IPv4 subnet mask consists of 32 bits; it is a sequence of ones 1 followed by a block of zeros 0. The ones indicate bits in the address used for the network prefix and the trailing block of zeros designates that part as being the host identifier.
The following example shows the separation of the network prefix and the host identifier from an address The operation is visualized in a table using binary address formats. The host part, which is , is derived by the bitwise AND operation of the address and the one's complement of the subnet mask. Subnetting is the process of designating some high-order bits from the host part as part of the network prefix and adjusting the subnet mask appropriately.
This divides a network into smaller subnets. The following diagram modifies the above example by moving 2 bits from the host part to the network prefix to form four smaller subnets each one quarter of the previous size. IPv4 uses specially designated address formats to facilitate recognition of special address functionality.
The first and the last subnets obtained by subnetting a larger network have traditionally had a special designation and, early on, special usage implications. The first subnet obtained from subnetting a larger network has all bits in the subnet bit group set to zero 0. It is therefore called subnet zero. It is therefore called the all-ones subnet. The IETF originally discouraged the production use of these two subnets. When the prefix length is not available, the larger network and the first subnet have the same address, which may lead to confusion.
Similar confusion is possible broadcast address at the end of the last subnet. Therefore, reserving the subnet values consisting of all zeros and all ones on the public Internet was recommended,  reducing the number of available subnets by two for each subnetting. This inefficiency was removed, and the practice was declared obsolete in and is only relevant when dealing with legacy equipment.
Although the all-zeros and the all-ones host values are reserved for the network address of the subnet and its broadcast address , respectively, in systems using CIDR all subnets are available in a subdivided network. Each broadcast address, i. The number of subnetworks available and the number of possible hosts in a network may be readily calculated.
For instance, the The highlighted two address bits become part of the network number in this process. The remaining bits after the subnet bits are used for addressing hosts within the subnet. In the above example, the subnet mask consists of 26 bits, making it The number of available subnets is 2 n , where n is the number of bits used for the network portion of the address. There is an exception to this rule for bit subnet masks,  which means the host identifier is only one bit long for two permissible addresses.
In such networks, usually point-to-point links , only two hosts the end points may be connected and a specification of network and broadcast addresses is not necessary. The design of the IPv6 address space differs significantly from IPv4.
The primary reason for subnetting in IPv4 is to improve efficiency in the utilization of the relatively small address space available, particularly to enterprises. No such limitations exist in IPv6, as the large address space available, even to end-users, is not a limiting factor.
It is used to route traffic between the global allocation spaces and within customer networks between subnets and the Internet at large. A compliant IPv6 subnet always uses addresses with 64 bits in the host identifier.
Although it is technically possible to use smaller subnets,  they are impractical for local area networks based on Ethernet technology, because 64 bits are required for stateless address autoconfiguration. IPv6 does not implement special address formats for broadcast traffic or network numbers,  and thus all addresses in a subnet are acceptable for host addressing. The all-zeroes address is reserved as the subnet-router anycast address.
From Wikipedia, the free encyclopedia. Logical subdivision of an IP network. For subnets in the mathematics of topology, see Subnet mathematics. See also: IPv4 subnetting reference. See also: IPv6 subnetting reference. Internet Standard Subnetting Procedure. RFC Updated by RFC Fuller; T. Li August Network Working Group. Braden, ed. October Requirements for Internet Hosts -- Communication Layers.
Narten; E. Nordmark; W. Simpson; H. Soliman September Singh; W. Beebee; E. Nordmark July Cisco Systems. Retrieved Traditionally, it was strongly recommended that subnet zero and the all-ones subnet not be used for addressing.
It is useful to preserve and extend the interpretation of these special addresses in subnetted networks.
This article is intended as a general introduction to the concepts of Internet Protocol IP networks and subnetting. A glossary is included at the end of article. These networks are arbitrarily defined into three main classes along with a few others that have predefined sizes. Each of them can be divided into smaller subnetworks by system administrators. A subnet mask is used to divide an IP address into two parts. One part identifies the host computer , the other part identifies the network to which it belongs.
You learn how to assign each interface on the router an IP address with a unique subnet. There are examples included in order to help tie everything together. The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared default configuration. If your network is live, make sure that you understand the potential impact of any command.
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A custom subnet mask borrows bits from the host portion of the address to create a subnetwork address between the network and host portions of an IP address. In.
A subnetwork or subnet is a logical subdivision of an IP network. Computers that belong to a subnet are addressed with an identical most-significant bit -group in their IP addresses. This results in the logical division of an IP address into two fields: the network number or routing prefix and the rest field or host identifier.
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