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Network Layer (Part V)

Network Layer (Part V). Network-to-Network Communications : ARP operation within a subnet. If a host wants to send data to another host, it must know the destination IP address.

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Network Layer (Part V)

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  1. Network Layer (Part V)

  2. Network-to-Network Communications:ARP operation within a subnet • If a host wants to send data to another host, it must know the destination IP address. • If it is unable to locate a MAC address for the destination in its own ARP table, the host initiates a process called an ARP request. • An ARP request enables it to discover the destination MAC address..

  3. Network-to-Network Communications:ARP operation within a subnet

  4. Network-to-Network Communications:ARP operation within a subnet

  5. Network-to-Network Communications:ARP operation within a subnet • A host builds an ARP request packet and sends it to all devices on the network. • To ensure that all devices see the ARP request, the source uses a broadcast MAC address. • The broadcast address in a MAC addressing scheme has all places set to hexadecimal F. • Thus, a MAC broadcast address would have the form FF-FF-FF-FF-FF-FF.

  6. Network-to-Network Communications:ARP operation within a subnet

  7. Network-to-Network Communications:ARP operation within a subnet • Because ARP request packets travel in a broadcast mode, all devices on the local network receive the packets and pass them up to the network layer for further examination. • If the IP address of a device matches the destination IP address in the ARP request, that device responds by sending the source its MAC address. • This is known as the ARP reply

  8. Network-to-Network Communications:ARP operation within a subnet

  9. Network-to-Network Communications:ARP operation within a subnet • Example:Source device 197.15.22.33 is asking for the MAC address of the destination with IP address 197.15.22.126,  Destination device 197.15.22.126 picks up the ARP request and responds with an ARP reply containing its MAC address. 

  10. Network-to-Network Communications:ARP operation within a subnet • Once the originating device receives the ARP reply, it extracts the MAC address from the MAC header, and updates its ARP table. • The originating device can then properly address its data with both, a destination MAC address, and a destination IP address. • It uses this new information to perform Layer 2 and Layer 3 encapsulations of the data, before it sends them out over the network.

  11. Network-to-Network Communications:ARP operation within a subnet • When the data arrives at the destination, the data link layer makes a match, strips off the MAC header, and transfers the data up to the network layer. • The network layer examines the data and finds that the IP address matches the destination IP address carried in the IP header. • The network layer strips off the IP header, and transfers the encapsulated data to the next highest layer in the OSI model, the transport layer (Layer 4). • This process is repeated until the rest of the packet's partially decapsulated data reaches the application, where the user data may be read.

  12. Advanced ARP Concepts: Default gateway • In order for a device to communicate with another device on another network, you must supply it with a default gateway. • A default gateway is the IP address of the interface on the router that connects to the network segment which the source host is located on. • The default gateway’s IP address must be in the same network segment as the source host.

  13. Advanced ARP Concepts: Default gateway • If no default gateway is defined, communication is possible only on the device’s own logical network segment. • The computer that sends the data does a comparison between the IP address of the destination and its own ARP table. • If it finds no match, it must have a default IP address to use. • Without a default gateway, the source computer has no destination MAC address, and the message is undeliverable.

  14. Advanced ARP Concepts: Problems with sending data to nodes on different subnets • One of the major problems in networking is how to communicate with devices that are not on the same physical network segment. • There are two parts to the problem. • The first is obtaining the MAC address of the destination host, and the second is transferring the data packets from one network segment to another, to get to the destination host.

  15. Advanced ARP Concepts: How ARP sends data to remote networks • ARP uses broadcast packets to accomplish its function. • Routers, however, do not forward broadcast packets. • In order for a device to send data to the address of a device that is on another network segment, the source device sends the data to a default gateway. • The default gateway is the IP address of the router interface that is connected to the same physical network segment as the source host. • The source host compares the destination IP address and its own IP address to determine if the two IP addresses are located on the same segment. • If the receiving host is not on the same segment, the source host sends the data to the default gateway.

  16. Advanced ARP Concepts: Proxy ARP • Proxy ARP is a variation of the ARP protocol. • In this case an intermediate device (e.g. router) sends an ARP response, on behalf of an end node, to the requesting host. • Routers running proxy ARP capture ARP packets. • They respond with their MAC addresses for those requests in which the IP address is not in the range of addresses of the local subnet.

  17. Advanced ARP Concepts: Proxy ARP • In the previous description of how data is sent to a host on a different subnet, the default gateway is configured. • If the source host does not have a default gateway configured, it sends an ARP request. • All hosts on the segment, including the router, receive the ARP request. • The router compares the IP destination address with the IP subnet address to determine if the destination IP address is on the same subnet as the source host. 

  18. Advanced ARP Concepts: Proxy ARP • If the subnet address is the same, the router discards the packet. • The reason that the packet is discarded is that the destination IP address is on the same segment as the source's IP address. • This means another device on the segment should respond to the ARP request. • The exception to this is that the destination IP address is not currently assigned, which will generate an error response on the source host.

  19. Advanced ARP Concepts: Proxy ARP • If the subnet address is different, the router will respond with its own MAC address for the interface that is directly connected to the segment on which the source host is located. • This is the proxy ARP. Since the MAC address is unavailable for the destination host, the router supplies its MAC address in order to get the packet. • Then the router can forward the ARP request (based on the destination IP address) to the proper subnet for delivery.

  20. Advanced ARP Concepts:Four Layer 3 flowcharts

  21. Advanced ARP Concepts:Four Layer 3 flowcharts • Create flowcharts for the following processes: • ARP • RARP • BOOTP • DHCP

  22. Routable Protocols: Routed protocols • IP is a network layer protocol, and because of that, it can be routed over an internetwork, which is a network of networks. • Protocols that provide support for the network layer are called routed or routable protocols.

  23. Routable Protocols:Other routed protocols • The focus of this course is on the most commonly used routable protocol, which is IP. • Even though you will concentrate on IP, it is important to know that there are other routable protocols. • Two of them are IPX/SPX and AppleTalk.

  24. Routable Protocols: Routable and non-routable protocols • Protocols such as IP, IPX/SPX and AppleTalk provide Layer 3 support and are, therefore, routable. • However, there are protocols that do not support Layer 3; these are classed as non-routable protocols. • The most common of these non-routable protocols is NetBEUI. • NetBEUI is a small, fast, and efficient protocol that is limited to running on one segment.

  25. Routable Protocols: Characteristics of a routable protocol • In order for a protocol to be routable, it must provide the ability to assign a network number, as well as a host number, to each individual device. • Some protocols, such as IPX, only require that you assign a network number; they use a host's MAC address for the physical number. • Other protocols, such as IP, require that you provide a complete address, as well as a subnet mask. • The network address is obtained by ANDing the address with the subnet mask.

  26. Routing Protocols:Examples of routing protocols • Routing protocols (Note: Do not confuse with routed protocols.) determine the paths that routed protocols follow to their destinations. • Examples of routing protocols include the Routing Information Protocol (RIP), the Interior Gateway Routing Protocol (IGRP), the Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First(OSPF).

  27. Routing Protocols:Examples of routing protocols • Routing protocols enable routers that are connected, to create a map, internally, of other routers in the network or on the Internet. • This allows routing (i.e. selecting the best path, and switching) to occur. Such maps become part of each router's routing table.

  28. Routing Protocols :Definition of routing protocol • Routers use routing protocols to exchange routing tables and to share routing information. • Within a network, the most common protocol used to transfer routing information between routers, located on the same network, is Routing Information Protocol (RIP).

  29. Routing Protocols :Definition of routing protocol • This Interior Gateway Protocol (IGP) calculates distances to a destination host in terms of how many hops (i.e. how many routers) a packet must pass through. • RIP enables routers to update their routing tables at programmable intervals, usually every 30 seconds. • One disadvantage of routers that use RIP is that they are constantly connecting to neighboring routers to update their routing tables, thus creating large amounts of network traffic.

  30. Routing Protocols :Definition of routing protocol • RIP allows routers to determine which path to use to send data. It does so by using a concept known as distance-vector. • Whenever data goes through a router, and thus, through a new network number, this is considered to be equal to one hop.

  31. Routing Protocols :Definition of routing protocol • A path which has a hopcount of four indicates that data traveling along that path would have to pass through four routers before reaching the final destination on the network. • If there are multiple paths to a destination, the path with the least number of hops would be the path chosen by the router.

  32. Routing Protocols :Definition of routing protocol • Because hop count is the only routing metric used by RIP, it doesn’t necessarily select the fastest path to a destination. • A metric is a measurement for making decisions. You will soon learn that other routing protocols use many other metrics besides hop count to find the best path for data to travel. • Nevertheless, RIP remains very popular, and is still widely implemented. • This may be due primarily to the fact that it was one of the earliest routing protocols to be developed

  33. Routing Protocols :Definition of routing protocol • One other problem posed by the use of RIP is that sometimes a destination may be located too far away to be reachable. • When using RIP, the maximum number of hops that data can be forwarded through is fifteen. • The destination network is considered unreachable if it is more than fifteen router hops away.

  34. Routing Protocols : Routing encapsulation sequence • At the data link layer, an IP datagram is encapsulated into a frame. • The datagram, including the IP header, is treated as data. • A router receives the frame, strips off the frame header, then checks the destination IP address in the IP header. • The router then looks for that destination IP address in its routing table, encapsulates the data in a data link layer frame, and sends it out to the appropriate interface. • If it does not find the destination IP address, it may drop the packet.

  35. Routing Protocols : Multi-protocol routing • Routers are capable of supporting multiple independent routing protocols, and of maintaining routing tables for several routed protocols, concurrently. • This capability allows a router to deliver packets from several routed protocols over the same data links.

  36. Other Network Layer Services :Connectionless network services • Most network services use a connectionless delivery system. • They treat each packet separately, and send it on its way through the network. • The packets may take different paths to get through the network, but are reassembled when they arrive at the destination. • In a connectionless system the destination is not contacted before a packet is sent

  37. Other Network Layer Services :Connectionless network services • A good analogy for a connectionless system is a postal system. • The recipient is not contacted before a letter is sent from one destination to another. • The letter is sent on its way, and the recipient learns of the letter when it arrives.

  38. Other Network Layer Services:Connection-oriented network services • In connection-oriented systems, a connection is established between the sender and the recipient before any data is transferred. • An example of a connection-oriented network is the telephone system. • You place a call, a connection is established, and then communication occurs.

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