Annex C08 OSI Network Layer

Question 0

Three (3) basic characteristics of Internet Protocol Version 4 (IPv4)

Answer 0

Connectionless: IP comm is unreliable like UDP, therefore, in order for packets to be sent reliably, IP relies on the layer 4 TCP connections, acknowledgments and error correction

Best Effort: No additional fail safes at the layer 3 level to guarantee delivery; thus, layer three simply uses best effort to get the packet from point a to point b

Media Independent Protocol: IP packets can be transmitted over cable, radio waves, fiber optic cable and other media. The protocol does not prefer any one type of media over another,it functions equally well over all of the different types of media

Question 1

Devices that operate the Network Layer of the OSI model

Answer 1

Main device used at the network layer is the router

Question 2

Actions that a router can take with a packet (1 of 3 actions)

Answer 2

1: If the network portion of the IP address matches one of the router’s directly connected networks, it will forward the packet out the proper interface
2: If it cannot match the network it will send the packet out to its default route if one is programmed
3: If it cannot match the network, and if the router does not have a default gateway programmed, it will drop the packet

Question 3

Maximum Transmission Unit (MTU)

Answer 3

Some networks have media restrictions and must enforce a MTU which limits the size of the PDU.

Question 4

Valid Internet Protocol Version 4 (IPv4) addresses

Answer 4

IPv4 written in dotted decimal notation. An IP address is composed of 4 binary octets totaling 32 bits or 4 bytes.

ex: 11001000.00010100.00010000.00010001

decimal form: 200.20.16.17

Question 5

Routing

Answer 5

Routers receive a layer 2 PDU, decapsulate the layer 2 header and trailer, making a forwarding decision based on the destination IP address in the layer 3 header, and then re encapsulating the PDU based on the outgoing interface and forwarding it

Question 6

Default Gateway

Answer 6

IP address used by a host to forward IP packets that have destination address outside of the local network. The default gateway address is the address of the router interface connected to the same local network as the source host. If the compter does not know the destination, it routes the packet out to the default gateway.

Question 7

Routing Table

Answer 7

Contains info about adj networks that allow the router to make a decision as to which interface to forward the packet to. The routing table is a list that a router holds in memory for the purpose of deciding how to forward packets.

Contains 3 important pieces of info to include destination networks, the metric, and the address of the next hop

Question 8

Static Route

Answer 8

Way of MANUALLY configuring the routing table in routers. A router can be programmed by a network administrator with all the routes that it needs to know about in order to do its job

Question 9

Dynamic Routing

Answer 9

Allows for routers to learn about other networks from other routers automatically. The routers share their networks with other routers automatically to build a complete topology of the network, and automatically adjusts to traffic changes

Question 10

Time to Live (TTL)

Answer 10

Describes the maximum number of hops the packet can take before it is considered “lost” or undeliverable. Each router that handles the packet decrements the TTL field by one. The packet will be dropped if the TTL value reaches zero. This keeps the internet from becoming cluttered by lost packets and creating such issues as routing loops.

Question 11

Metric

Answer 11

Calculated number that is used to determine which route to a destination network is best in the event that the router knows about multiple paths to the same network. Lower metrics indicate that the cost to reach a network through that path is lower, and therefore preferred.