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Network Layer and Routing The Network Layer Layer 3 on the OSI reference model The layer at which routing occurs! It responds to service requests from the transport layer and issues service requests to the data link layer. The network layer addresses messages and translates logical addresses and names into physical addresses. OSI In an OSI network there are four significant architectural entities: hosts, areas, a backbone, and a domain. A domain is any portion of an OSI network that is under common administrative authority. Within any OSI domain, one or more areas can be defined. An area is a logical entity; it is formed by a set of contiguous routers and the data links that connect them. All routers in the same area exchange information about all of the hosts that they can reach. OSI & Routing Areas are connected to form a backbone. All routers on the backbone know how to reach all areas. Equipment A router consists of a computer networking device that determines the next network point to which to forward a data packet toward its destination, a process known as routing. Routing occurs at layer 3 of the OSI model. Routers Routing is most commonly associated with the Internet Protocol, although other less-popular routed protocols continue in use. In the original 1960’s-era of routing, generalpurpose computers served as routers. Although general-purpose computers can perform routing, modern high-speed routers are highly specialised computers, generally with extra hardware added to accelerate both common routing functions such as packet forwarding and specialised functions such as IPSec encryption. Routers A router must be connected to at least two networks, or it will have nothing to route. A special variety of router is the one-armed router used to route packets in a virtual LAN environment. A router creates and/or maintains a table, called a "routing table" that stores the best routes to certain network destinations and the "routing metrics" associated with those routes. Switch A device that filters and forwards packets between LAN segments. Switches operate at the data link layer (layer 2) and sometimes the network layer (layer 3) of the OSI model and therefore support any packet protocol. LANs that use switches to join segments are called switched LANs or, in the case of Ethernet networks, switched Ethernet LANs. Network Layer … It also determines the route from the source to the destination computer and manages traffic problems, such as switching, routing, and controlling the congestion of data packets. The network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. Network Layer… The Network layer performs network routing, flow control, segmentation/desegmentation, and error control functions. The network layer deals with transmitting information all the way from its source to its destination - and transmitting things from anywhere you like, to anywhere you like. Network Layer… If you can't contact a place at the network layer, then you can't contact that place at all. However, it does it in a very basic way, without error detection or flow control. The following slide indicates particular items the network layer addresses. Network Layer Responsibilities Is the network connection orientated or connectionless? For example, snail mail is connectionless, because you can send a letter to someone completely out of the blue. On the other hand, the telephone system is connection orientated, because you have to close to one half and make a connection before you can talk to them. Network Layer Responsibilities What are the Global Addresses? Everybody in the network needs to have a unique address which determines who they are. This address will normally be hierarchical, so you can be “Jane Smith" to Londoners, or “Jane Smith, London" to people in the UK, or “Jane Smith, London, UK" to people anywhere in the world. On the internet, these addresses are known as IP Numbers. Network Layer Responsibilities How do you forward a message? This is of particular interest to mobile applications, where a user may rapidly move from place to place, and it must be arranged that his messages follow him. IPv4 doesn't really allow for this, though it has been hacked on somewhat at a later date. Fortunately, the forthcoming IPv6 has a much better designed solution, which should make these kinds of applications much smoother. Network Layer Protocols IP IPX NW Link NetBEIU Routing-switches A switch that also performs routing operations. Usually a switch operates at layer 2 (the Data Link layer) of the OSI model while routers operate at layer 3 (the Network layer). Routing switches perform many of the layer 3 functions usually reserved for routers. The routing is implemented in hardware rather than software, it is faster. The downside of routing switches is that they are not as powerful or as flexible as full-fledged routers. Because they perform some layer 3 functions, routing switches are sometimes called layer-3 switches. Routing Routing within an area is called level 1 routing; routing between areas is called level 2 routing. An IS that can route only within areas is known as a level 1 IS. A level 1 IS needs to know only about the other level 1 ISs in its own level 1 area and about the nearest level 2 IS that it can use to forward traffic out of its own area. Routing from Cisco To help simplify, Cisco has created route maps on their routers. Over time, Cisco has enhanced its route redistribution support to improve administrative control over methods by which routing information moves between routing domains. A route map is a set of instructions that tell the router how routing information is to be redistributed between two routing protocols or between two instances of the same routing protocol. Routing from Cisco Route maps give network managers unprecedented control over the ways that routing information is propagated in their networks. Redistribution configuration files that use route maps are easy to create, understand, and modify. Using route maps, Cisco users are able to build larger, more robust, reliable networks, with better traffic control than ever before. IPv4 IPv4 is version 4 of IP. It was the first version of the Internet Protocol to be widely deployed, and forms the basis for the current (as of 2004) Internet. IPv4 addresses are written in dot-decimal notation. Here's an example: 127.0.0.1. IPv4 uses 32-bit addresses, limiting it to 4,294,967,296 unique addresses, many of which are reserved for special purposes such as local networks or multicast addresses, reducing the number of addresses that can be allocated as public Internet addresses. IPv4 As the number of addresses available is consumed, an IPv4 address shortage appears to be inevitable in the long run. This limitation has helped stimulate the push towards IPv6, which is currently in the early stages of deployment, and is expected to eventually replace IPv4. IPv6 IPv6 is version 6 of the Internet Protocol. IPv6 is intended to replace the previous standard, IPv4, which only supports up to about 4 billion (4 × 109) addresses, whereas IPv6 supports up to about 3.4 × 1038 addresses. IPv6 IPv6 is the second version of the Internet Protocol to be widely deployed, and is expected to form the basis for future expansion of the Internet. The compelling reason behind the formation of IPv6 was lack of address space, especially in the heavily populated countries of Asia such as India and China. IPv6 Notation IPv6 addresses are 128 bits long but are normally written as eight groups of 4 hexadecimal digits each. For example, 3ffe:6a88:85a3:08d3:1319:8a2e:0370:7344 IPv6 Notation If a 4 digit group is 0000, it may be omitted. For example: 3ffe:6a88:85a3:0000:1319:8a2e:0370:7344 is the same IPv6 address as 3ffe:6a88:85a3::1319:8a2e:0370:7344