SEMESTER 1 Chapter 5
OSI Network Layer

 V 4.0

What services does the network layer provide?

services to exchange the individual pieces of data over the network between identified end devices.


What are the 4 basic processes used at the network layer?






What is the PDU of the network layer?



What information is contained in the layer 3 PDU?

Source and Destination IP address


What is the role of a router?

to select paths for and direct packets toward their destination. This process is known as routing


What is it called when a packet passed through an intermediary device?



What happens to the transport layer PDU as the packet moves through the network?

It stays the same


What does the network layer protocol specify?

the packet structure and processing used to carry the data from one host to another host

List the 5 Network layer protocols.

Internet Protocol version 4 (IPv4)

Internet Protocol version 6 (IPv6)

Novell Internetwork Packet Exchange (IPX)


Connectionless Network Service (CLNS/DECNet)


Which protocol will be the focus of this class?


What are the basic characteristics of IPv4?

Connectionless - No connection is established before sending data packets.

Best Effort (unreliable) - No overhead is used to guarantee packet delivery.

Media Independent - Operates independently of the medium carrying the data.


Describe what a  connectionless service in networking means?

requires no initial exchange of control information to establish an end-to-end connection before packets are forwarded, nor does it require additional fields in the PDU header to maintain this connection


What is the main issue that may occur in a connectionless exchange?

Connectionless packet delivery may, however, result in packets arriving at the destination out of sequence. If out-of-order or missing packets create problems for the application using the data, then upper layer services will have to resolve these issues.


What does unreliable mean in networking?

Unreliable means simply that IP does not have the capability to manage, and recover from, undelivered or corrupt packets.


Which layer is responsible for adding reliability to packet delivery?

Transport Layer


Describe media independent.

IPv4 and IPv6 operate independently of the media that carry the data at lower layers of the protocol stack.


What layer is responsible for preparing a packet for the media?

Physical Layer


What is the one characteristic of the network layer that is dependent on the media?

Maximum Transmission Unit MTU


What portion of the IP packet does a router use to make forwarding decisions?



What remains untouched as packets travel through the network?

In all cases, the data portion of the packet - that is, the encapsulated Transport layer PDU - remains unchanged during the Network layer processes.

What are the 6 key fields in an IP packet?

IP Source Address

IP Destination Address

Time-to-Live (TTL)

Type-of-Service (ToS)


Fragment Offset


What happens to a packet if the TTL field reaches 0?

It is discarded

What is the checksum field used for?

To determine if the header has reached the destination intact.


What are three methods of determining how to divide your network?

Geographic location



Why would you group users geographically?

To improve manageability and network performance.


Why would you group users by task?

To improve performance based on network requirements of each task.


Why would you group networks by ownership?

To maintain control and limit access.


What are the 3 main issues as networks grow larger?

Performance degradation

Security issues

Address Management


What is a network broadcast?

A broadcast is a message sent from one host to all other hosts on the network.


What is broadcast domain?

The network area in which all hosts receive broadcasts


Is it good or bad to increase the number of broadcast domains in your network?



What is the major advantage of dividing networks by ownership?

access to and from resources outside each network can be prohibited, allowed, or monitored


What efficiency does dividing large networks into separate networks provide to the hosts?

hosts who need to communicate are grouped together reduces the unnecessary overhead of all hosts needing to know all addresses.


What is the device called that hosts send information to that must travel outside their network?



What is hierarchical addressing?

A hierarchical address uniquely identifies each host. It also has levels that assist in forwarding packets across internetworks, which enables a network to be divided based on those levels.


What is the example presented that describes hierarchical addressing in our daily lives?

Zip codes and house addresses


How many bits are in an IPv4 address?



How is the ip address divided?

IPv4 addresses are divided in four groups of eight bits (octets)


What portion of the IP address does a router use?



What is the actual address used for a default gateway?

The router interface IP address at the edge of the network.


What is the route that the router forwards a packet to called?

Next-hop address


What decisions do routers make as a packet enters an interface?

Is the network directly connected or not. Yes forward to that network. No forward to the next-hop router.


How is the default gateway configured on a host running windows?

On a Windows computer, the Internet Protocol (TCP/IP) Properties tools are used to enter the default gateway IPv4 address


What portion of the IP address must be the same for the host and gateway?

Network and Subnetwork

What command is issued at the command prompt of windows to see the default gateway setting?


What is it called when a router makes forwarding decisions?



What is the map called that a router uses to make forwarding decisions (Hint: look at the figure)?

Routing Table


What does a routing table contain?

The routing table stores information about connected and remote networks.


What are the 3 main features of routes in a routing table?

Destination network




What happens if the packet does not have an address contained in the routing table?

The packet is dropped


What is a default route used for?

The default route is used when the destination network is not represented by any other route in the routing table.

What is the command to view the routing table for a host?



What is the destination network IP address and subnet mask for the default network interface?


What three commands are listed that allow you to change the routing table?

route ADD

route DELETE

route CHANGE


What does the address in the routing table represent?

represents a range of host addresses and sometimes a range of network and host addresses


If more than one route exists in the routing table for an address, which one will be selected?

The route that is most specific for that IP address

When is the default route used?

When the  IP address does not match any other entry in the routing table


What is the next-hop?

the address of the device that will process the packet next


Why does a directly connected device not have a next-hop?

There are no intermediary devices and the packet is directly forwarded

What are the three decisions a router can make about a packet?

Forward it to the next-hop router

Forward it to the destination host

Drop it


Which layer does a router process packets at?



What layer is the packet encapsulated at before it leaves the router?

Data Link Layer 2

What is another name for a default gateway?

Gateway of last resort


Why would a packet take a different route than another packet that is part of the same data stream?

Rotuers may learn a new path between packets


Why are default routes so important to routers?

because the gateway router is not likely to have a route to every possible network on the Internet.

What happens if a router has not route for the packet and does not have a default route?

The packet is dropped


Why doesn’t IP have error notification?

It would reduce efficiency and add overhead


What does a router need to forward a packet?



What happens if routing table are not up to date?

that packets may not be forwarded to the most appropriate next-hop, causing delays or packet loss


What are two ways a router can learn about routes?

route information can be manually configured on the router or learned dynamically from other routers in the same internetwork


What is a static route?

A manually configured route


What must be done if the internetwork changes?

Static routes must be reconfigured


What is the definition of a dynamic routing protocol?

set of rules by which routers dynamically share their routing information


What happens when a router receives information about changes in routes?

It updates its’ routing table


What are 3 common routing protocols?

Routing Information Protocol (RIP)

Enhanced Interior Gateway Protocol (EIGRP)

Open Shortest Path First (OSPF)


What are the 2 costs listed with using a dynamic protocol?

Overhead that consumes network bandwidth

Processing capacity necessary


What advantages are listed for static routing?

No network overhead produced

No processing capacity used

Administrative distance is configured to ensure efficient and effective routing