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Embed code for: 13.3 Layer 3 Addressing
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13.3 Layer 3 Addressing
Layer 3 Addressing Section 13: Exploring the Packet Delivery Process The network layer provides connectivity and path selection between two IP hosts that may be on the same or separate networks. At the boundary of each local network, an intermediary network device, usually a router, deencapsulates the frame to read the destination IP address that is contained in the IP header of the packet (the Layer 3 PDU). The TCP/IP Internet layer is usually described in terms of OSI model Layer 3. OSI Layer 3 is the network layer. The figure represents the hierarchy of the TCP/IP protocol stack. Communications on a TCP/IP network begin with an application that needs to send data. The chunk of data is encapsulated into a transport layer protocol header that conforms to the the type of delivery service that is required. TCP provides connectionoriented, guaranteed delivery of data, while UDP provides connectionless, besteffort services. The transport header is then encapsulated into an IP header with the information that is necessary to route the packet from source to destination. The IP header is then encapsulated in a Layer 2 Ethernet frame for transmission across the local LAN. Routers must have a way of determining which addresses are on a given network. The network portion of the IP address is determined by analyzing the subnet mask. In this example, 24 bits of the 32bit IP address (/24) are used to indicate the Network ID, and 8 bits are used for unique hosts on this network. The /24 notation is equivalent to the subnet mask 255.255.255.0. Routers use the network portion of the destination IP address to determine the best path (or paths) to reach the destination IP host. Once the path is determined, the router encapsulates the packet in a new Layer 2 frame and sends it toward the destination end device. Destination IP Addresses Interface Destination IP Network Interface Destination IP Network GigabitEthernet 1/0 192.168.16.0/24 GigabitEthernet 1/1 192.168.17.0/24 Layer 3 addresses are assigned to end devices, such as hosts, and to network devices that provide Layer 3 functions. Routers have their own Layer 3 address on each interface. Every network device that provides a Layer 3 function maintains a Layer 3 address table. Because the Layer 2 MAC address of a device is local to the LAN segment, it does not cross Layer 3 boundaries. When data needs to be routed to a different network, the source MAC address is replaced with the MAC address of the router interface that is attached to the new network. In the example, IP host 192.168.16.21 is sending a packet to IP host 192.168.18.25. Each router in the path from the source to the destination strips the Layer 2 MAC address and replaces it with its own MAC address. The Layer 3 source and destination IP have not been modified in transit. When IP host 192.168.18.25 responds to source IP host 192.168.16.21, MAC address deencapsulation and re encapsulation would occur in the reverse order on the path. Layer 3 Addressing Up Next: Address Resolution Protocol