SEMESTER 1 Chapter 5
OSI Network Layer
V 4.0
5.1.1.1 / 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? / Addressing
Encapsulation
Routing
Decapsulation
What is the PDU of the network layer? / Packet
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? / Hop
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
5.1.1.2 / List the 5 Network layer protocols. / Internet Protocol version 4 (IPv4)
Internet Protocol version 6 (IPv6)
Novell Internetwork Packet Exchange (IPX)
AppleTalk
Connectionless Network Service (CLNS/DECNet)
Which protocol will be the focus of this class? / IP
5.1.2.1 / 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.
5.1.3 / 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.
5.1.4 / 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
5.1.5 / 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
5.1.6 / What portion of the IP packet does a router use to make forwarding decisions? / Header
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.
5.1.7.1 / What are the 6 key fields in an IP packet? / IP Source Address
IP Destination Address
Time-to-Live (TTL)
Type-of-Service (ToS)
Protocol
Fragment Offset
What happens to a packet if the TTL field reaches 0? / It is discarded
5.1.7.2 / What is the checksum field used for? / To determine if the header has reached the destination intact.
5.2.1 / What are three methods of determining how to divide your network? / Geographic location
Purpose
Ownership
5.2.1.2 / 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.
5.2.2 / 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? / Good
5.2.3 / What is the major advantage of dividing networks by ownership? / access to and from resources outside each network can be prohibited, allowed, or monitored
5.2.4 / 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? / Gateway
5.2.5 / 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
5.2.5 / How many bits are in an IPv4 address? / 32
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? / Network
5.3.1 / 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
5.3.2 / 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.
5.3.3 / 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
5.3.3.2 / What command is issued at the command prompt of windows to see the default gateway setting? / ipconfig
5.3.3.3 / What is it called when a router makes forwarding decisions? / Routing
What is the map called that a router uses to make forwarding decisions (Hint: look at the figure)? / Routing Table
5.3.4 / 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
Next-hop
Metric
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.
5.3.4.2 / What is the command to view the routing table for a host? / Netstat
What is the destination network IP address and subnet mask for the default network interface? / 0.0.0.0 0.0.0.0
What three commands are listed that allow you to change the routing table? / route ADD
route DELETE
route CHANGE
5.3.5 / 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
5.3.5.2 / When is the default route used? / When the IP address does not match any other entry in the routing table
5.3.6 / 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
5.3.7.1 / 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? / 3
What layer is the packet encapsulated at before it leaves the router? / Data Link Layer 2
5.3.7.2 / 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.
5.3.7.3 / 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
5.4.1 / What does a router need to forward a packet? / Next-hop
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
5.4.2 / What is a static route? / A manually configured route
What must be done if the internetwork changes? / Static routes must be reconfigured
5.4.3 / 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
* Networks can be grouped based on factors
–Geographic location
–Purpose
–Ownership
* Common issues with large networks are
–Performance degradation
–Security issues
–Address Management
* You should still be expected to know what is broadcast domain and know how to count them?
* The role of the Network layer is to transfer data from the host that originates the data to the host that uses it.
–If the destination host is in the same network,
–the packet is delivered between the two hosts on the local media without the need for a router.
–If communication is between hosts in different networks,
–The local network delivers the packet from the source to its gateway router.
–The router examines the network portion of the packet destination address and forwards the packet to the appropriate interface.
–If the destination network is (NOW) directly connected to this router, the packet is forwarded directly to that host.
–If the destination network is not directly connected, the packet is forwarded on to a second router.
* What information is added during the encapsulation of the OSI network layer?
–When referring to the Network layer, we call this PDU a packet.
–The address of the host to which it is being sent. This address is referred to as the destination address.
–The address of the originating host is called the source address.
*IPv4 basic characteristics:
–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.
* Time-to-Live
–The Time-to-Live (TTL) indicates the remaining "life" of the packet.
–The TTL value is decreased by at least one each time the packet is processed by a router (that is, each hop).
–When the value becomes zero, the router discards or drops the packet and it is removed from the network data flow.
–This mechanism prevents packets that cannot reach their destination from being forwarded indefinitely between routers in a routing loop.
–Decrementing the TTL value at each hop ensures that it eventually becomes zero and that the packet with the expired TTL field will be dropped.
* To support data communications between networks over internetworks, Network layer addressing schemes are hierarchical.
- Layer 3 addresses supply the network portion of the address. Routers forward packets between networks by referring only to the part of the Network layer address that is required to direct the packet toward the destination network.
- By the time the packet arrives at the destination host network, the whole destination address of the host will have been used to deliver the packet.
* What is the purpose of “default gateway” address configured on the PC? What address is used to configure the “default gateway” address.
- Keep in mind that it is not feasible for a particular host to know the address of every device on the Internet with which it may have to communicate.
–To communicate with a device on another network, a host uses the address of this gateway, or default gateway, to forward a packet outside the local network.
–This gateway address is the address of a router interface that is connected to the same network as the host.
* As each packet arrives at a router, the destination network address is examined and compared to the routes in the routing table. What are the ways that you can configure the “routing table”?
- Routes to remote networks with the associated next hops can be manually configured on the router.
- This is known as static routing.
- Routing protocols are the set of rules by which routers dynamically share their routing information.
- As routers become aware of changes to the networks for which they act as the gateway, or changes to links between routers, this information is passed on to other routers.
- the exchange of route information adds overhead that consumes network bandwidth
- provide routers with up-to-date routing tables
* What is the purpose of “default route” in a router?
- if the routing table does not contain a more specific route entry for an arriving packet, the packet is forwarded to the interface indicated by a default route, if one exists.
–The default route is also known as the Gateway of Last Resort.
–This process may occur a number of times until the packet reaches its destination network.
–If a route to a destination network does not exist, the packet cannot be forwarded.
- [Tony]: Dropped the packet
* Although, we only cover briefly about the ‘routing table”. You are expected to know what does it looks like and how to read the router table and know what information are included in the routing table?
- a routing table have three main features:
–Destination network
–Metric
–Next-hop
- A next-hop is the address of the device that will process the packet next.
- For a host on a network, the address of the default gateway (router interface) is the next-hop for all packets destined for another network.
- In the routing table of a router, each route lists a next hop for each destination address that is encompassed by the route.
* Which intermediate devices can be configured security and protect the network?
–Security between networks is implemented in an intermediary device (a router or firewall appliance) at the perimeter of the network.
* You should know how to read the output shown below. Based on the output shown below, which address is the PC IP address and which address is the default gateway address? What is your subnet mask?