Chapter 6

De-encapsulation

(Way up) When the packet arrives at the NETWORK layer of the destination host, the host checks the IP header of the packet. If the destination IP address within the header MATCHES its own IP address, the IP header is removed from the packet. After the packet is de-encapsulated by the network layer, the resulting Layer 4 PDU is passed up to the appropriate service at the transport layer.

Encapsulation on the network layer

(way down) The network layer encapsulates the PDU from the transport layer into a packet. The encapsulation process adds IP header INFORMATION, such as the IP address of the source (sending) and destination (receiving) hosts.

Can you explain each of the four processes that the network layer uses to achieve end to end transport of packets?

ADDRESSING, ENCAPSULATION, ROUTING, DE-ENCAPLSUATION Addressing end devices (configured with an IP address), Encapsulation(adds IP source and destination, encapsulates PDU into a packet), Routing(selects the best path to the destination and direct them to the destination) , De-encapsulating (removing the IP addresses and pass up to the transport layer)

To accomplish end-to-end transport, the network layer uses four basic processes:

Addressing end devices, Encapsulation, Routing and De-encapsulation ERDA

The routing table of a router can store information about:

Directly-connected routes, Remote routes and Default route

Addressing end devices

End devices must be configured with a unique IP address for identification on the network.

A host can send a packet to:

Itself(ping itself) , Local host (same network address) and remote host (not same network address)

Default route

Like a host, routers also use a default route as a LAST RESORT if there is no other route to the desired network in the routing table.

nonvolatile memory that stores the startup configuration of a device

NVRAM

Network layer

Provides services to allow end devices to EXCHANGE data across the network and direct packets between hosts.

Host Routing Tables

ROUTE PRINT or NETSTAT -R command can be used to display the host routing table. displays three sections related to the current TCP/IP network connections: INTERFACE LIST (MAC), IPV4 AND 6 ROUTE TABLE. Interface list (MAC address and interface number of wifi and internet).

Default Gateway

Routes traffic to other networks. Has a local IP address in the same address range as other hosts on the network. Can take data in and forward data out. The host receives the IPv4 address of the default gateway either dynamically from DHCP or configured manually.

IPv4 Router Routing Table

SHOW IP ROUTE command on router. shows networks connected and HOW THE ROUTE WAS LEARNED. If the destination network MATCHES a route in the routing table, the router FORWARDS the packet using the information specified in the routing table. When a packet arrives at the router interface, the router examines the packet header to determine the destination network. If the destination network MATCHES a route in the routing table, the router FORWARDS the packet using the information specified in the routing table. If there are two or more possible routes to the same destination, the metric is used to decide which route appears in the routing table.

Remote routes

These routes come from remote networks connected to other routers. Routes to these networks can be MANUALLY configured on the local router by the network administrator or dynamically configured by enabling the local router to exchange routing information with other routers using a DYNAMIC protocol.

Directly-connected routes

These routes come from the ACTIVE router interfaces. Routers add a directly connected route when an interface is configured with an IP address and is activated. Each of the router's interfaces is connected to a different network segment.

Router Packet Forwarding Decision

When a host sends a packet to another host, it will use its routing TABLE to determine where to SEND the packet. If the destination host is on a remote network, the packet is forwarded to the DEFUALT GATEWAY.

ROM

a device that allows the devices on one network to communicate with devices on other networks

Transport layer

manages the DATA transport between the PROCESSES running on each host

a value that is used by a routing protocol to identify the best route to insert into a routing table

metric

flash

nonvolatile memory that stores the IOS and other system files

Can you explain what VLSM is and why we use it instead of classful addressing?

not equal size. we use it so that we don't have any wasted addresses.

A host can ping itself by ...

sending a packet to a special IPv4 address of 127.0.0.1, which is referred to as the loopback interface. Pinging the loopback interface tests the TCP/IP protocol stack on the host.

intermediate device (WAP or LAN switch )

several wired and wireless devices connected using an intermediate device . provides INTERCONNECTIONS between local hosts on the local network. Local hosts can reach each other and share INFORMATION without the need for any additional devices. If a host is sending a packet to a device that is configured with the same IP network as the host device, the packet is simply forwarded out of the host interface, through the intermediate device, and to the destination device directly.

next-hop

the destination router indicated by the outgoing interface entry in the routing table

maximum transmission unit

the largest PDU that is supported

Routing

the process of identifying the best path for a destination