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59-599: XIO is usually used in a star topology, using a router ASIC called Crossbow (Xbow) to connect up to eight fully symmetrical devices in a system (one of them is usually the memory controller / CPU bridge, called HEART in Octane or Hub in Origin). Other devices known to have XIO interfaces are: The XIO employs two source-synchronous channels (one in each direction), each 8 or 16 bits wide. They are clocked at 400 MHz to achieve peak rates of 800 MB/s (i.e. in megabytes). Each of

118-653: A switching node using software and an interface computer were first proposed by Donald Davies for the NPL network in 1966. The same idea was conceived by Wesley Clark the following year for use in the ARPANET , which were named Interface Message Processors (IMPs). The first interface computer was implemented at the National Physical Laboratory in the United Kingdom in early 1969, followed later that year by

177-497: A wide area network (WAN), so they may have considerable memory installed, multiple WAN interface connections, and substantial onboard data processing routines. They may also provide connectivity to groups of file servers or other external networks. In enterprises, a core router may provide a collapsed backbone interconnecting the distribution tier routers from multiple buildings of a campus, or large enterprise locations. They tend to be optimized for high bandwidth but lack some of

236-410: 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 an IP network have at least one network address . Usually, this address is unique to each device and can either be configured automatically by a network service with

295-617: A limiting factor. As in IPv4, subnetting in IPv6 is based on the concepts of variable-length subnet masking (VLSM) and the Classless Inter-Domain Routing methodology. 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. Given

354-414: A security feature by all experts. Some experts argue that open source routers are more secure and reliable than closed source routers because errors and potentially exploitable vulnerabilities are more likely to be discovered and addressed in an open-source environment. Routers are also often distinguished on the basis of the network in which they operate. A router in a local area network (LAN) of

413-446: 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

472-561: A single organization is called an interior router . A router that is operated in the Internet backbone is described as exterior router . While a router that connects a LAN with the Internet or a wide area network (WAN) is called a border router , or gateway router . Routers intended for ISP and major enterprise connectivity usually exchange routing information using the Border Gateway Protocol (BGP). RFC   4098 defines

531-451: 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

590-402: A subdivided network. For example, a / 24 network can be divided into sixteen usable / 28 networks. Each broadcast address, i.e. *.15 , *.31 , …, *.255 , reduces only the host count in each subnets. 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

649-400: A subnet are acceptable for host addressing. The all-zeroes address is reserved as the subnet-router anycast address. The subnet router anycast address is the lowest address in the subnet, so it looks like the “network address”. If a router has multiple subnets on the same link, then it has multiple subnet router anycast addresses on that link. The first and last address in any network or subnet

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708-418: A variety of sources, such as a default or static routes that are configured manually, or dynamic entries from routing protocols where the router learns routes from other routers. A default route is one that is used to route all traffic whose destination does not otherwise appear in the routing table; it is common – even necessary – in small networks, such as a home or small business where

767-468: Is a computer and networking device that forwards data packets between computer networks , including internetworks such as the global Internet . A router is connected to two or more data lines from different IP networks . When a data packet comes in on a line, the router reads the network address information in the packet header to determine the ultimate destination. Then, using information in its routing table or routing policy , it directs

826-440: Is a logical subdivision of an IP network . The practice of dividing a network into two or more networks is called subnetting . Computers that belong to the same subnet are addressed with an identical group of its most-significant bits of 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

885-495: Is an identifier for a specific host or network interface. The routing prefix may be expressed as the first address of a network, written in Classless Inter-Domain Routing (CIDR) notation, followed by a slash character ( / ), and ending with the bit-length of the prefix. For example, 198.51.100.0 / 24 is the prefix of the Internet Protocol version 4 network starting at the given address, having 24 bits allocated for

944-406: Is divided into two logical parts, the network prefix and the host identifier . All hosts on a subnet 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

1003-738: Is in common use. Some routers can connect to Data service units for T1 connections via serial ports. The hierarchical internetworking model divides enterprise networks into three layers: core, distribution, and access. Access routers, including small office/home office (SOHO) models, are located at home and customer sites such as branch offices that do not need hierarchical routing of their own. Typically, they are optimized for low cost. Some SOHO routers are capable of running alternative free Linux-based firmware like Tomato , OpenWrt , or DD-WRT . Distribution routers aggregate traffic from multiple access routers. Distribution routers are often responsible for enforcing quality of service across

1062-588: Is probable that XIO uses STL (which SGI likes to call SGI transistor logic ) low-voltage single-ended I/O standard. CrossTown is a version of XIO utilizing PECL for differential I/O standard (like NUMAlink ) for longer connections. Higher-level encapsulation differs from NUMAlink (like used to connect HUB chips in Origin-series machines). It is well suited for short memory transactions. XIO uses very delicate compression connectors, which should be handled with extreme care. Router (computing) A router

1121-408: 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 IP address. For example, the prefix 198.51.100.0 / 24 would have the subnet mask 255.255.255.0 . Traffic is exchanged between subnets through routers when the routing prefixes of the source address and

1180-413: Is the simplest and most easily implemented: the router simply drops new incoming packets once buffer space in the router is exhausted. RED probabilistically drops datagrams early when the queue exceeds a pre-configured portion of the buffer, until reaching a pre-determined maximum, when it drops all incoming packets, thus reverting to tail drop. WRED can be configured to drop packets more readily dependent on

1239-482: Is used to enable data packets to be forwarded from one transmission system to another. Routers may also be used to connect two or more logical groups of computer devices known as subnets , each with a unique network prefix . Routers may provide connectivity within enterprises, between enterprises and the Internet, or between internet service providers ' (ISPs') networks, they are also responsible for directing data between different networks. The largest routers (such as

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1298-495: Is written as 255.255.255.0 . The modern standard form of specification of the network prefix is CIDR notation, used for both IPv4 and IPv6. It counts the number of bits in the prefix and appends that number to the address after a slash (/) character separator. This notation was introduced with Classless Inter-Domain Routing (CIDR). In IPv6 this is the only standards-based form to denote network or routing prefixes. For example,

1357-642: The Cisco CRS-1 or Juniper PTX) interconnect the various ISPs, or may be used in large enterprise networks. Smaller routers usually provide connectivity for typical home and office networks. All sizes of routers may be found inside enterprises. The most powerful routers are usually found in ISPs, academic and research facilities. Large businesses may also need more powerful routers to cope with ever-increasing demands of intranet data traffic. A hierarchical internetworking model for interconnecting routers in large networks

1416-647: The Dynamic Host Configuration Protocol (DHCP), manually by an administrator, or automatically by the operating system with stateless address autoconfiguration . An address fulfills the functions of identifying the host and locating it on the network in destination routing. The most common network addressing architecture is Internet Protocol version 4 (IPv4), but its successor, IPv6 , has been increasingly deployed since approximately 2006. An IPv4 address consists of 32 bits. An IPv6 address consists of 128 bits. In both architectures, an IP address

1475-506: The International Network Working Group (INWG). These gateway devices were different from most previous packet switching schemes in two ways. First, they connected dissimilar kinds of networks, such as serial lines and local area networks . Second, they were connectionless devices, which had no role in assuring that traffic was delivered reliably, leaving that function entirely to the hosts . This particular idea,

1534-402: The all ones host address, i.e. the last address within a network, for broadcast transmission to all hosts on the link. The first subnet obtained from subnetting a larger network has all bits in the subnet bit group set to zero. It is therefore called subnet zero . The last subnet obtained from subnetting a larger network has all bits in the subnet bit group set to one. It is therefore called

1593-418: 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 with the 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

1652-623: The end-to-end principle , was pioneered in the CYCLADES network. The idea was explored in more detail, with the intention to produce a prototype system as part of two contemporaneous programs. One was a program at Xerox PARC to explore new networking technologies, which produced the PARC Universal Packet system. Some time after early 1974, the first Xerox routers became operational. Due to corporate intellectual property concerns, it received little attention outside Xerox for years. The other

1711-449: The optical fiber lines of the Internet backbone . Routers can be built from standard computer parts but are mostly specialized purpose-built computers . Early routers used software -based forwarding, running on a CPU . More sophisticated devices use application-specific integrated circuits (ASICs) to increase performance or add advanced filtering and firewall functionality. When multiple routers are used in interconnected networks,

1770-763: The IMPs at the University of California, Los Angeles , the Stanford Research Institute , the University of California, Santa Barbara , and the University of Utah School of Computing in the United States. All were built with the Honeywell 516 . These computers had fundamentally the same functionality as a router does today. The idea for a router (called a gateway at the time) initially came about through an international group of computer networking researchers called

1829-497: The IPv4 network 192.0.2.0 with the subnet mask 255.255.255.0 is written as 192.0.2.0 / 24 , and the IPv6 notation 2001:db8:: / 32 designates the address 2001:db8:: and its network prefix consisting of the most significant 32 bits. 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

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1888-435: 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. In addition, IPv4 uses

1947-531: The address size of 128 bits, it therefore has a /64 routing prefix. 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 . The Internet Engineering Task Force recommends the use of / 127 subnets for point-to-point links, which have only two hosts. IPv6 does not implement special address formats for broadcast traffic or network numbers, and thus all addresses in

2006-443: 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 ( 192.0.2.130 ) and its associated / 24 subnet mask ( 255.255.255.0 ). The operation is visualized in a table using binary address formats. The result of the bitwise AND operation of IP address and

2065-427: 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 a locally connected network or a remote network. The subnet mask of

2124-508: The default route simply sends all non-local traffic to the Internet service provider . The default route can be manually configured (as a static route); learned by dynamic routing protocols; or be obtained by DHCP . A router can run more than one routing protocol at a time, particularly if it serves as an autonomous system border router between parts of a network that run different routing protocols; if it does so, then redistribution may be used (usually selectively) to share information between

2183-492: 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, efficient allocation of address space is necessary. Subnetting may also enhance routing efficiency, or have advantages in network management when subnets are administratively controlled by different entities in

2242-475: The destination is not needed, and is generally not known to a router. For IPv6, however, on-link determination is different in detail and requires the Neighbor Discovery Protocol (NDP). IPv6 address assignment to an interface carries no requirement of a matching on-link prefix and vice versa, with the exception of link-local addresses . Since each locally connected subnet must be represented by

2301-469: The devices can utilize the full bandwidth, as the XBow router prevents collisions by being able to route between any two points. Transfer is organized into micropackets. These contain a total of 128 bits of data and 32 bits of control. The control information encapsulates an 8 bit sideband (used by higher layers for framing), sequence numbers (for go-back-n link-layer retransmissions) and check bits ( CRC -16). It

2360-402: The different protocols running on the same router. Besides deciding to which interface a packet is forwarded, which is handled primarily via the routing table, a router also has to manage congestion when packets arrive at a rate higher than the router can process. Three policies commonly used are tail drop , random early detection (RED), and weighted random early detection (WRED). Tail drop

2419-416: The features of edge routers. External networks must be carefully considered as part of the overall security strategy of the local network. A router may include a firewall , VPN handling, and other security functions, or they may be handled by separate devices. Routers also commonly perform network address translation which restricts connections initiated from external connections but is not recognized as

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2478-399: The following four / 26 subnets. 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 255.255.255.192, leaving 6 bits for the host identifier. This allows for 62 host combinations (2 −2). In general,

2537-423: The layer-3 IP packet, specifically the destination IP address. When a router receives a packet, it searches its routing table to find the best match between the destination IP address of the packet and one of the addresses in the routing table. Once a match is found, the packet is encapsulated in the layer-2 data link frame for the outgoing interface indicated in the table entry. A router typically does not look into

2596-416: 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

2655-505: The mid-1970s and in the 1980s, general-purpose minicomputers served as routers. Modern high-speed routers are network processors or highly specialized computers with extra hardware acceleration added to speed both common routing functions, such as packet forwarding, and specialized functions such as IPsec encryption. There is substantial use of Linux and Unix software-based machines, running open source routing code, for research and other applications. The Cisco IOS operating system

2714-443: The network prefix, and the remaining 8 bits reserved for host addressing. Addresses in the range 198.51.100.0 to 198.51.100.255 belong to this network, with 198.51.100.255 as the subnet broadcast address . The IPv6 address specification 2001:db8:: / 32 is a large address block with 2 addresses, having a 32-bit routing prefix. For IPv4, a network may also be characterized by its subnet mask or netmask , which

2773-489: The number of available hosts on a subnet is 2 −2, where h is the number of bits used for the host portion of the address. The number of available subnets is 2 , where n is the number of bits used for the network portion of the address. There is an exception to this rule for 31-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 endpoints) may be connected and

2832-471: The packet payload, but only at the layer-3 addresses to make a forwarding decision, plus optionally other information in the header for hints on, for example, quality of service (QoS). For pure IP forwarding, a router is designed to minimize the state information associated with individual packets. Once a packet is forwarded, the router does not retain any historical information about the packet. The routing table itself can contain information derived from

2891-475: The packet to the next network on its journey. Data packets are forwarded from one router to another through an internetwork until it reaches its destination node . The most familiar type of IP routers are home and small office routers that forward IP packets between the home computers and the Internet. More sophisticated routers, such as enterprise routers, connect large business or ISP networks to powerful core routers that forward data at high speed along

2950-585: The routers can exchange information about destination addresses using a routing protocol . Each router builds up a routing table , a list of routes, between two computer systems on the interconnected networks. The software that runs the router is composed of two functional processing units that operate simultaneously, called planes : A router may have interfaces for multiple types of physical layer connections, such as copper cables, fiber optic , or wireless transmission. It can also support multiple network layer transmission standards. Each network interface

3009-449: The routing table when a packet forwarding decision is made. Some of the functions may be performed through an application-specific integrated circuit (ASIC) to avoid the overhead of scheduling CPU time to process the packets. Others may have to be performed through the CPU as these packets need special attention that cannot be handled by an ASIC. Subnets A subnetwork , or subnet ,

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3068-423: The subnet mask is the network prefix 192.0.2.0 . The host part, which is 130 , is derived by the bitwise AND operation of the address and the ones' 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

3127-441: 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

3186-411: The type of traffic. Another function a router performs is traffic classification and deciding which packet should be processed first. This is managed through QoS , which is critical when Voice over IP is deployed, so as not to introduce excessive latency . Yet another function a router performs is called policy-based routing where special rules are constructed to override the rules derived from

3245-440: The types of BGP routers according to their functions: Wi-Fi routers combine the functions of a router with those of a wireless access point . They are typically devices with a small form factor, operating on the standard electric power supply for residential use. Connected to the Internet as offered by an Internet service provider , they provide Internet access through a wireless network for home or office use. The concepts of

3304-408: Was independently designed. Major router operating systems, such as Junos and NX-OS , are extensively modified versions of Unix software. The main purpose of a router is to connect multiple networks and forward packets destined either for directly attached networks or more remote networks. A router is considered a layer-3 device because its primary forwarding decision is based on the information in

3363-436: Was led by William Yeager and MIT's by Noel Chiappa . Virtually all networking now uses TCP/IP, but multiprotocol routers are still manufactured. They were important in the early stages of the growth of computer networking when protocols other than TCP/IP were in use. Modern routers that handle both IPv4 and IPv6 are multiprotocol but are simpler devices than ones processing AppleTalk, DECnet, IPX, and Xerox protocols. From

3422-421: Was recommended, reducing the number of available subnets by two for each subnetting. This inefficiency was removed, and the practice was declared obsolete in 1995 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

3481-765: Was the DARPA -initiated program, which created the TCP/IP architecture in use today. The first true IP router was developed by Ginny Travers at BBN , as part of that DARPA-initiated effort, during 1975–1976. By the end of 1976, three PDP-11 -based routers were in service in the experimental prototype Internet. Mike Brecia, Ginny Travers, and Bob Hinden received the IEEE Internet Award for early IP routers in 2008. The first multiprotocol routers were independently created by staff researchers at MIT and Stanford in 1981 and both were also based on PDP-11s. Stanford's router program

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